Source: Intermediate Microeconomics, Florida International University
Difficulty: Intermediate | Prerequisites: Production theory (production functions, MP, AP, returns to scale).
Cost theory translates production relationships into money terms. Once you know how inputs map to output, the next question is what it costs. From there, you can work out how firms behave in different market structures: perfect competition (many price-taking firms), monopoly (one firm with market power), and the pricing strategies monopolists use to extract surplus. This material ties together everything from consumer theory through to welfare analysis.
Costs come in several types: accounting, opportunity, economic, and sunk. In the short run, firms face fixed and variable costs, and the shapes of MC and ATC curves drive production decisions. Perfectly competitive firms are price takers who break even when P = ATC. Monopolists have market power and can price discriminate in several ways to capture consumer surplus.
Accounting costs
Actual, explicit expenses to produce output: buying a tractor, paying rent, purchasing materials.
Opportunity costs
The value of the next best alternative forgone. In simple terms, what are you giving up? Implicit costs that do not appear on a balance sheet.
Economic costs
Accounting costs plus opportunity costs. This is the full cost as economists see it.
Sunk costs
Expenses already made that cannot be recovered. These should be ignored when making decisions, because the money is gone regardless. Think of it as: you already paid, so only future costs and benefits matter.
Fixed costs (FC)
Costs that do not change with output in the short run. Rent, insurance, equipment leases.
Variable costs (VC)
Costs that change with output in the short run. Labour, raw materials, energy.
Total cost (TC)
TC = FC + VC.
Marginal cost (MC)
The change in total cost from producing one more unit. MC = ΔTC / ΔQ. Often considered the most important cost concept in economic analysis.
Average total cost (ATC)
Total cost per unit of output. ATC = TC / Q.
Average fixed cost (AFC)
Fixed cost per unit. AFC = FC / Q. Always falls as output rises.
Average variable cost (AVC)
Variable cost per unit. AVC = VC / Q.
Breakeven point
The output level where P = ATC. The firm makes zero economic profit. Above this, economic profits are positive.
Economies of scope
When producing two goods together is cheaper than producing each separately. C(q1, q2) ≤ C(q1) + C(q2).
1st degree price discrimination
Charging each consumer their exact willingness to pay. Extracts all consumer surplus.
2nd degree price discrimination
Charging different prices based on quantity purchased. Examples: bulk discounts, block pricing on mobile data plans.
3rd degree price discrimination
Dividing consumers into groups and charging each group a different price. Examples: student discounts, senior pricing.
Two-part tariff
A pricing structure with an entry fee plus a per-unit charge. Think of amusement park admission plus ride tickets.
Peak load pricing
Adjusting prices by time period (hourly, daily, seasonally) to reflect changes in demand. Surge pricing on ride-share apps.
Marginal external benefit (MEB)
The additional benefit to third parties when output increases by one unit. Relevant to externalities and social welfare.
Accounting costs are the explicit, out-of-pocket expenses: what you actually pay for inputs.
Opportunity costs are implicit. They capture what you give up. What are you giving up to attend university? To go to class today? These do not show up on financial statements but are essential for economic decision-making.
Economic costs = accounting costs + opportunity costs. This is the measure economists care about.
Sunk costs are money already spent that cannot be recovered. The rational approach is to ignore them in future decisions. Example: you bought an app for coursework, then a better app is released. The rational choice is to buy the better one if its benefits exceed its cost, regardless of what you already spent.
In the short run, some costs are fixed and some are variable.
Fixed costs (FC): cannot be adjusted in the short run.
Variable costs (VC): adjustable in the short run.
Total costs: TC = VC + FC.
Cost equations:
Marginal cost: MC = ΔC(q) / Δq
Average total cost: ATC = C(q) / q (always positive, can rise or fall)
Average fixed cost: AFC = FC / q
Average variable cost: AVC = VC / q
Key relationships:
MC intersects AVC at AVC's minimum. This parallels how MP intersects AP at AP's maximum.
MC = w / MP(L). When marginal product rises, marginal cost falls, and vice versa.
AVC = w / AP(L).
As output rises, AFC continuously falls (spreading fixed costs over more units), while AVC eventually rises due to diminishing returns.
In the long run, all inputs are adjustable. The firm's problem mirrors the consumer's:
Instead of a budget constraint, firms choose an isocost line (by choosing inputs).
Instead of reaching the highest indifference curve, firms try to operate on the lowest isocost line for a desired output.
The optimal point is where the isoquant is tangent to the isocost line: MRTS = w/r.
Economies of scope: producing two related goods together can be cheaper than producing each separately. C(q1, q2) ≤ C(q1) + C(q2). Diseconomies of scope are the reverse.
Dynamic cost changes: over time, firms may become better at making their product, causing long-run average cost to fall. This is a learning effect, distinct from economies of scale. An example is aircraft manufacturing, where each successive contract tends to be cheaper as the firm gains experience.
Four assumptions define perfect competition:
Homogeneous good: Firm A's product is a perfect substitute for Firm B's product. No product differentiation.
Price takers: firms cannot affect the market price. They take it as given. If they raise their price even slightly, they lose all customers.
Perfect information: both consumers and producers have full information. No asymmetric information (no moral hazard, no adverse selection).
Free entry and exit: no barriers to entry (no patents, no licensing requirements). Firms can enter or exit the market freely in the long run.
Breakeven point: when P = ATC, the firm makes zero economic profit. When P > ATC, the firm earns positive economic profits. In the long run, free entry drives economic profits to zero.
When can a firm price discriminate? Several conditions must hold simultaneously:
Monopoly conditions are satisfied (unique good, one seller, barriers to entry).
Consumers cannot resell the good to each other.
Different buyers must have different elasticities. If everyone has the same price sensitivity, there is no way to charge different prices.
1st degree price discrimination: charge each consumer their exact willingness to pay (WTP). MR becomes identical to the demand curve. The firm extracts all consumer surplus. This is the theoretical extreme and rarely achieved in practice.
2nd degree price discrimination: charge different prices based on quantity purchased. The consumer self-selects into a pricing tier.
Per-unit examples: small vs large coffee.
Block pricing examples: tiered data charges on a mobile plan, utility bills with usage brackets.
3rd degree price discrimination: divide consumers into identifiable groups and charge each group a different price. Student discounts, senior pricing, and regional pricing are all examples.
Intertemporal price discrimination: charge different prices based on time of purchase. New releases are priced high and drop over time, capturing high-WTP buyers first.
Two-part tariffs: charge one fee for access (the entry fee) and a separate fee per unit consumed. Amusement parks, warehouse club memberships, and mobile contracts with a base fee plus per-minute charges all follow this structure.
Peak load pricing: adjust prices by time period to reflect demand fluctuations. Prices may shift seasonally, daily, hourly, or by the second. Ride-share surge pricing and off-peak electricity rates are familiar examples.
Marginal external benefit (MEB): the additional benefit accrued to others (not the buyer or seller) when output increases by one unit. Relevant when markets produce less than the socially optimal quantity because private decisions ignore these external benefits.
Constrained maximisation note: when solving optimisation problems, if the solution falls outside the feasible range, take the boundary value. For instance, if the constraint is 0 ≤ x ≤ 5 and your unconstrained solution gives x = 8, the answer is x = 5.
Quotient rule reminder (for derivatives of fractions): if F(x) = G(x) / H(x), then F'(x) = [G'(x) * H(x) - G(x) * H'(x)] / [H(x)]².
Students often treat sunk costs as relevant to future decisions. They are not. The money is already gone. Only future costs and benefits should inform a choice.
A frequent mistake is thinking that zero economic profit means the firm is failing. Zero economic profit means the firm is covering all costs, including opportunity costs. It is earning a normal return.
Students sometimes confuse the three degrees of price discrimination. First degree is individual pricing (charge each person their WTP). Second degree is quantity-based (bulk discounts). Third degree is group-based (student vs adult pricing). The numbering does not indicate severity or importance.
Some students assume that a monopolist can charge any price it likes. A monopolist still faces a demand curve. It chooses the price-quantity combination that maximises profit, but it cannot sell at a high price and a high quantity simultaneously.
⚠️ Know the distinction between accounting costs, opportunity costs, and economic costs. Exam questions often ask you to calculate economic profit, which requires including opportunity costs.
⚠️ Be able to identify the breakeven point (P = ATC) and shutdown point (P = AVC) on a graph.
⚠️ MC intersects AVC at AVC's minimum. This is a classic graphical exam question.
⚠️ Be ready to identify which degree of price discrimination is being described in a scenario. The conditions for price discrimination (monopoly power, no resale, different elasticities) are frequently tested.
⚠️ The four assumptions of perfect competition are a standard short-answer or multiple-choice topic.
⚠️ The cost-minimisation condition MRTS = w/r (isoquant tangent to isocost) appears in long-run cost questions.
Fill in the blank: Economic costs = ______ costs + ______ costs. (accounting, opportunity)
True or False: Sunk costs should be considered when making future decisions. (False)
Fill in the blank: TC = ______ + ______ . (FC + VC)
True or False: In perfect competition, firms are price takers. (True)
Fill in the blank: 1st degree price discrimination extracts all ______ . (consumer surplus)
Q: You bought an app for $30 to help with coursework. A better app is released for $20. Should you switch?
A: Yes, if the new app's benefits exceed its $20 cost. The $30 already spent is a sunk cost and should not factor into the decision.
Q: Name the four assumptions of perfect competition.
A: Homogeneous goods (no product differentiation), price-taking firms, perfect information (no asymmetric information), and free entry and exit (no barriers).
Q: A mobile phone company charges $10 per month plus $0.05 per text message. What pricing strategy is this?
A: A two-part tariff. The $10 is the entry (access) fee and the $0.05 per text is the per-unit charge.
Q: A cinema charges $8 for adults and $5 for students. What degree of price discrimination is this?
A: Third degree. Consumers are divided into identifiable groups (adults vs students) and charged different prices.
Q: At what point does a perfectly competitive firm break even?
A: Where P = ATC. The firm makes zero economic profit, meaning it covers all costs including opportunity costs.
Cost curves are built directly from the production function: the MC and AVC shapes mirror the MP and AP shapes from production theory. The breakeven and shutdown conditions in perfect competition connect to welfare analysis and the concept of economic profit. Price discrimination strategies reappear in industrial organisation and game theory, and the conditions for price discrimination (market power, segmentation) link back to elasticity. The marginal external benefit concept bridges into externalities and public goods.
Tags: accounting costs, opportunity costs, economic costs, sunk costs, fixed costs, variable costs, total cost, marginal cost, MC, average total cost, ATC, average variable cost, AVC, average fixed cost, AFC, breakeven point, shutdown point, perfect competition, price taker, homogeneous good, free entry and exit, perfect information, monopoly, price discrimination, first degree, second degree, third degree, two-part tariff, peak load pricing, intertemporal pricing, block pricing, economies of scope, diseconomies of scope, isoquant, isocost, MRTS, marginal external benefit, MEB, constrained maximisation, quotient rule, intermediate microeconomics