Build the ownership cost boundary

Fixed costs

Registration, base insurance, subscriptions and storage can continue even when the asset is idle. They matter most at low utilization.

Variable costs

Fuel, consumables, transaction fees and wear can be linked to miles, hours or cycles. Use a physical driver rather than a vague annual average.

A second car driven 4,000 miles per year

Cost layer
Registration and insurance are mostly fixed
Time or usage input
Fuel and tires vary with miles
Cost layer
Depreciation depends on age and mileage
Time or usage input
A major repair is a probability, not a monthly invoice
Cost layer
Cost per mile rises when fixed costs are spread over low use

The boundary should include only costs that change between the options being compared.

This decision belongs in The Real Cost of Ownership. Use Total Cost of Ownership: A Reusable Model for Any Purchase to check the next assumption, then test the numbers in the Repair vs. Replace Calculator.

Adjust the ledger for the real decision

Step costs

Capacity upgrades, service tiers and major scheduled maintenance jump at thresholds instead of moving smoothly with each unit of use.

Risk costs

Repairs, deductibles and downtime are uncertain. Show probability-weighted expected cost and a cash reserve requirement separately because averages do not pay a sudden bill.

Build a complete cost boundary

Fixed, variable and uncertain costs need different treatment. Add fixed obligations directly, connect variable cost to usage and show risk as both an expected cost and a possible cash requirement.

Evidence for each cost behavior

  • Contracted costs that continue even when usage falls.
  • Unit price and measurable driver for variable spending.
  • Failure range, likelihood evidence and maximum affordable loss.
  • Assign each expense a cost driver.
  • Calculate a base usage case.
  • Run low and high utilization.
  • Separate expected risk cost from required liquidity.
  • Identify thresholds where step costs appear.

Turn the cost boundary into a working estimate

Repair vs. Replace Calculator

Put two imperfect options on an annualized cost basis without counting sunk cost.

Costs that disappear from a monthly view

  • Dividing every annual cost evenly and calling it variable.
  • Using expected repair cost as the needed emergency reserve.
  • Ignoring idle-period fixed expenses.
  • Assuming depreciation is only time-based.

Questions this decision usually raises

Is depreciation fixed or variable?

Both time and use can affect it; model them explicitly when material.

How do I price a rare failure?

Use probability times loss for comparison, then separately assess whether you can absorb the full event.

Why separate step costs?

They reveal breakpoints where a different product or service plan becomes economical.

Sources for prices, timing and useful life

This page explains a calculation, not a personal recommendation. Contract rules and consumer protections vary by product and jurisdiction. Check the current agreement and official guidance before acting.