Relevant Costs
Future differential costs that matter — filter out the noise so only decision-changing numbers remain.
Why This Matters
Managers constantly make choices: accept a special order, drop a product line, outsource a function, add a shift. The financial question behind all of them is: "How will this decision change our future cash flows and profits?"
Not all costs are equally useful for answering that question. Some costs will be the same no matter which option you choose. Others will change depending on the decision. Only the latter are relevant.
Definition: Relevant vs. Irrelevant Costs
A relevant cost (or benefit) has two characteristics:
1. Future-oriented
It has not yet been incurred. Past spending is sunk — only forward-looking cash flows matter.
2. Differs between alternatives
It changes depending on which option you choose. If both paths share the same cost, it's irrelevant.
Irrelevant Examples
- → Sunk costs — already incurred, not recoverable
- → Committed fixed costs unchanged by the decision (e.g., rent you keep either way)
- → Allocated overhead that doesn't change in total
Relevant Examples
- → Additional materials and labor for a special order
- → Incremental shipping for an overseas customer
- → Avoidable fixed costs if shutting down a product line
- → Opportunity costs — benefits given up by choosing one path
Sunk Costs: The Most Common Trap
A sunk cost is a cost that has already been incurred and cannot be changed by any current or future decision.
CLASSIC TRAP
"We can't drop this product now; we've already invested $200,000 in development!"
The $200,000 is gone either way. The real question: "From today forward, will continuing this product add more cash than it consumes?"
Sunk costs are always irrelevant. Ignore them and look only at future incremental revenue and cost.
Avoidable vs. Unavoidable Costs
When comparing alternatives — especially make-or-buy and drop-or-keep decisions — focus on avoidable costs.
ABC Coffee Shop — Drop Pastries?
Avoidable (Relevant)
- → Pastry ingredients
- → Pastry baker's wages
- → Pastry-specific packaging
- → Pastry display electricity (if case turned off)
- → Pastry-specific marketing
Unavoidable (Irrelevant)
- → Portion of rent
- → General manager salary
- → Base utilities for the shop
Only the avoidable portion of overhead matters for the decision.
Opportunity Cost
Opportunity cost is the benefit you give up by choosing one alternative over another. It is often the most important relevant cost — even though it's never recorded in the accounting system.
Scarce kitchen capacity
Using oven time for a low-margin product may block production of a high-margin product. The lost profit on the high-margin product is an opportunity cost of the low-margin choice.
ABC back room decision
If ABC uses its back room for storage instead of renting it out at $800/month, the forgone rental income is an opportunity cost of using the space — relevant even though it never hits the income statement.
Relevant Cost Format: Comparative Analysis
Relevant cost decisions are best structured as side-by-side comparisons that include only amounts that differ.
KEEP VS. DROP PASTRIES — ANNUAL INCREMENTAL IMPACT
| Keep | Drop | Difference | |
|---|---|---|---|
| Pastry contribution margin | $30,000 | $0 | ($30,000) |
| Avoidable pastry fixed costs | ($12,000) | $0 | +$12,000 |
| Change in operating income | ($18,000) | ||
Dropping pastries reduces operating income by $18,000. Irrelevant: allocated rent, manager salary. You don't rebuild a full income statement — only the differences.
Common Irrelevant Costs That Sneak In
Allocated overhead
Company-wide overhead allocated to products (e.g., 30% of HQ costs). The allocation may change on paper if a product is dropped, but the total HQ cost often remains — making it irrelevant.
Depreciation
For an existing asset, depreciation is a sunk cost allocation. The original purchase is sunk; future depreciation typically doesn't change with make-buy or keep-drop unless you dispose of the asset.
Book value
Book value of an asset is sunk. The disposal value (what you can sell it for today) and any gain/loss related to disposal are relevant.
Steps for Relevant Cost Analysis
- 1
Define the decision clearly
Accept or reject the special order? Make or buy the part? Keep or drop the product?
- 2
Identify alternatives
At least two; sometimes more.
- 3
List all future revenues and costs for each alternative
Include potential opportunity costs.
- 4
Eliminate sunk costs and costs that do not differ
These are irrelevant; removing them simplifies the analysis.
- 5
Compare only the remaining (relevant) amounts
Compute the incremental impact on operating income.
- 6
Consider qualitative factors
Employees, quality, customer relationships, supplier reliability, strategic positioning.
Relevant vs. Irrelevant Classifier
Classify each ABC Coffee Shop scenario as Relevant or Irrelevant — covering sunk costs, opportunity costs, and differential scenarios. Then reveal the answer.
1.$40,000 paid for a label-printing press two years ago (ABC Coffee Shop)
Hint type: Sunk cost
2.$200,000 already spent on R&D for a new pastry line that may be discontinued
Hint type: Sunk cost
3.Extra coffee beans and cups needed to fulfill a 500-cup/week special order
Hint type: Differential (incremental)
4.Corporate HQ overhead allocated at 30% of revenue (total HQ cost unchanged if product dropped)
Hint type: Allocated overhead
5.Forgone $1,200/year from tasting events if barista time stays tied to label printing
Hint type: Opportunity cost
6.Lost profit on 4,000 regular cold brew sales ($2.20) displaced by a special order at $1.70
Hint type: Opportunity cost
7.Monthly shop rent ($1,000) when deciding whether to drop the pastry line (shop stays open)
Hint type: Unavoidable fixed
8.Pastry baker's wages ($2,400/month) if ABC drops the pastry line and releases the baker
Hint type: Avoidable fixed
9.Book value ($8,000) of old espresso equipment being considered for replacement
Hint type: Sunk cost
10.Current resale value ($3,500) of the old espresso machine if sold today
Hint type: Differential
11.Depreciation expense on existing label press (no plan to dispose; make-or-buy decision only)
Hint type: Sunk allocation
12.Incremental international shipping ($400) if ABC accepts an overseas bulk order
Hint type: Differential (incremental)
Key Takeaway
Relevant costs are future costs and benefits that differ between alternatives. Sunk costs, committed fixed costs that don't change with the decision, and most allocated overhead are irrelevant. Good decision analysis strips away all irrelevant amounts, focuses on incremental revenues, incremental costs, and opportunity costs, and then layers in qualitative considerations before choosing a course of action.
Test Your Understanding
Sunk costs, allocated overhead, and opportunity costs — check your answers below.
Question 1: The $75,000 you spent on a machine two years ago is:
Question 2: True or False: Allocated corporate overhead is always relevant to product-line drop decisions.
Question 3: ABC Coffee Shop can rent its back room for $800/month or use it for storage. The forgone rental income when choosing storage is:
Ready to Practice?
Classify costs, strip sunk amounts, and build incremental analyses in the Practice Lab.
Try the Practice LabWhat's Next?
Next module: Make or Buy Decision — using relevant cost analysis to decide whether to continue producing a component in-house or outsource it to a supplier.
Make or Buy Decision
Outsourcing analysis with opportunity cost
Fixed vs. Variable Costs
Foundation for incremental analysis