Difficulty: Intermediate | Prerequisites: Basic cost behaviour (variable vs. fixed costs), contribution margin concept.
Incremental analysis compares only the costs and revenues that change between two alternatives, ignoring everything that stays the same. For sell-or-process-further decisions, you compare the extra revenue from additional processing against the extra cost. For special orders, you compare the offered price to the relevant (usually variable) costs per unit, and accept if the price covers those costs and the company has spare capacity.
Relevant cost
A future cost that differs between decision alternatives. Only relevant costs matter for a decision. In simple terms, if a cost stays the same no matter what you choose, you can ignore it.
Incremental revenue
The additional revenue earned by choosing one alternative over another. Think of it as the extra money that comes in because you picked Option B instead of Option A.
Incremental cost
The additional cost incurred by choosing one alternative over another. This is the flip side of incremental revenue: the extra spending triggered by the decision.
Sunk cost
A cost already incurred that cannot be recovered regardless of any future decision. In simple terms, money already spent is gone, so it should never influence what you do next.
Contribution margin
Selling price minus variable costs per unit. This is the amount each unit "contributes" toward covering fixed costs and generating profit.
Special order
A one-off order from a customer, usually at a price below the normal selling price. The key question is whether the order covers its own variable costs and whether the company has the capacity to fill it without disrupting regular sales.
Excess capacity
Unused production capacity that the company has no current plans to use. When a special order uses excess capacity, fixed costs are irrelevant because they are already being incurred regardless.
Sell-or-process-further decision
The choice between selling a product in its current state or processing it further to sell at a higher price. The rule: process further only if the incremental revenue exceeds the incremental cost of additional processing.
The decision framework is straightforward: compare the incremental revenue to the incremental cost of further processing. If the extra revenue exceeds the extra cost, process further. If not, sell as-is.
The original production costs are sunk at the point of this decision. They have already been incurred and do not change regardless of what you do next. Only the additional costs and additional revenues matter.
Worked example: Simon Corp.
Simon makes unassembled chairs, selling price $20, product cost $8
The product line manager proposes selling assembled chairs instead
Assembled chair market price: $25
Cost of assembly: $4 per chair
Analysis:
Incremental revenue: $25 - $20 = $5 per chair
Incremental cost: $4 per chair
Net benefit of processing further: $5 - $4 = $1 per chair
The original $8 product cost is irrelevant here. It has already been spent whether Simon sells the chair assembled or unassembled.
Conclusion: Simon should sell the assembled chair because the $5 in incremental revenue exceeds the $4 in incremental costs.
A special order is typically evaluated when the company has excess (unused) capacity. The decision rule: accept the order if the special price exceeds the relevant costs per unit.
When excess capacity exists, fixed costs are irrelevant. They are already being incurred and will not change whether the order is accepted or rejected. Only variable costs are relevant.
If accepting the order would require displacing regular sales or incurring additional fixed costs, those become relevant too. Context matters.
Worked example: Joseph Company
Per-unit costs: $11 variable, $4 fixed (allocated), total $15
Normal selling price: $24
Katherine offers to buy 2,500 units at $16 each
Units would come from excess capacity with no alternative use
Analysis:
Special order price per unit: $16
Relevant cost per unit (variable only): $11
Contribution per unit from special order: $16 - $11 = $5
Total additional profit: $5 x 2,500 = $12,500
The $4 in allocated fixed costs per unit is irrelevant. These costs exist regardless of whether the order is accepted. They are allocated to each unit for product costing purposes, but they do not change with this decision.
Conclusion: Joseph should accept Katherine's offer. Each unit sold increases profit by $5.
Students often include sunk costs in their analysis. Once a cost has been incurred, it is irrelevant to any future decision. The original production cost in a sell-or-process-further problem is a sunk cost.
Students frequently include allocated fixed costs when evaluating special orders that use excess capacity. Allocated fixed costs do not change with the decision and should be excluded. Only variable costs are relevant when spare capacity exists.
Students sometimes compare the special order price to the normal selling price and reject orders below normal price. The correct comparison is against the relevant (variable) cost per unit, not the normal price.
Students occasionally confuse incremental revenue with total revenue. In a sell-or-process-further decision, incremental revenue is the difference between the two selling prices, not the higher selling price itself.
⚠️ Exam questions nearly always test whether you can identify which costs are relevant. The most common trap is an answer option that includes fixed costs in a special order with excess capacity.
⚠️ Sell-or-process-further questions will often present total costs and total revenues for each alternative. Strip these back to incremental figures before deciding.
⚠️ Watch for answer options that present the correct decision but the wrong reasoning. "Sell the assembled chair because revenues are $5 higher" is incomplete: revenues are $5 higher and costs are $4 higher, so the net benefit is $1. The correct reasoning compares incremental revenue to incremental cost.
⚠️ Special order questions may introduce capacity constraints or opportunity costs. If the order displaces regular sales, the lost contribution margin from those displaced sales becomes a relevant cost.
True or false: In a sell-or-process-further decision, the original production cost is a relevant cost. (False. It is a sunk cost.)
Fill in the blank: When a special order uses excess capacity, only ______ costs are relevant. (Variable.)
True or false: A special order should be rejected if the price is below the normal selling price. (False. It should be accepted if the price exceeds the relevant variable cost per unit and capacity exists.)
Fill in the blank: Incremental revenue equals the difference between ______ and ______. (The higher selling price and the lower selling price, i.e. the two alternatives.)
True or false: Allocated fixed costs per unit are relevant to a special order decision when the company has spare capacity. (False. They do not change with the decision.)
Q: A company sells a product for $30. It could process the product further at a cost of $7 and sell it for $40. Should it process further?
A: Yes. Incremental revenue is $40 - $30 = $10. Incremental cost is $7. Since $10 > $7, the net benefit is $3 per unit.
Q: A company has per-unit costs of $14 variable and $6 fixed (allocated). Normal selling price is $28. A one-off customer offers $17 per unit for 1,000 units using excess capacity. Should the company accept?
A: Yes. The relevant cost is the $14 variable cost. The special order price of $17 exceeds the variable cost by $3 per unit, adding $3,000 in profit. The $6 fixed cost is allocated and does not change.
Q: Using the same scenario above, what if the company had to turn away regular customers to fill this order? How does the analysis change?
A: The lost contribution margin from displaced regular sales becomes a relevant cost. If regular sales contribute $28 - $14 = $14 per unit, the opportunity cost of each displaced unit is $14. The special order contributes only $3 per unit, so the company would lose $11 per unit on each displaced sale. In that case, reject the order.
Q: A company can sell a partially finished widget for $50 or complete it for an extra $18 and sell it for $63. The original manufacturing cost was $35. What is the net benefit of processing further?
A: Incremental revenue: $63 - $50 = $13. Incremental cost: $18. Net effect: $13 - $18 = -$5. The company should sell as-is. The original $35 manufacturing cost is sunk and irrelevant.
This material connects directly to segment elimination and make-or-buy decisions, which use the same relevant cost framework but apply it to different questions (drop a business unit, or outsource a component). The underlying logic is identical: isolate the costs and revenues that change, ignore the ones that do not.
Contribution margin analysis, covered in cost-volume-profit (CVP), is the foundation for special order decisions. If you are comfortable with CVP, special orders are a natural extension.
Opportunity cost, introduced here in the context of excess capacity, also appears in capital budgeting and constrained-resource decisions (e.g. which product to prioritise when machine hours are limited).
Relevant costs, differential costs, incremental analysis, sell or process further, further processing decision, special order, one-time order, excess capacity, idle capacity, sunk cost, contribution margin, variable costing, opportunity cost, short-term decision making, managerial accounting, cost accounting, ACCY, intermediate financial accounting