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2.7Cost Analysis: Cost Per Test Type
Cost Model
| Cost Factor | Unit Test | Integration Test | E2E Test |
|---|---|---|---|
| Write time | 10-30 min | 30-60 min | 1-4 hours |
| Execution time | < 1 second | 1-10 seconds | 30 seconds - 5 minutes |
| Maintenance per year | Low (rarely breaks) | Medium (breaks on API changes) | High (breaks on UI changes) |
| Infrastructure cost | None (runs anywhere) | Low (needs test DB or mocks) | High (needs browser, servers, data) |
| Flakiness risk | Very low | Low | High |
| Debugging time on failure | 5 minutes (pinpointed) | 15 minutes (narrowed to boundary) | 30-60 minutes (could be anywhere) |
ROI Comparison
Assume a bug in the checkout flow that could be caught at any level:
Unit test:
Write: 20 min, Run: 0.5s, Maintain: 5 min/month, Debug on fail: 5 min
Annual cost: ~2 hours
Integration test:
Write: 45 min, Run: 5s, Maintain: 15 min/month, Debug on fail: 15 min
Annual cost: ~4 hours
E2E test:
Write: 2 hours, Run: 2 min, Maintain: 45 min/month, Debug on fail: 45 min
Annual cost: ~12 hours
The E2E test costs 6x more annually than the unit test. If the bug can be caught at the unit level, the E2E test is a waste of resources. But if the bug is specifically about how the checkout page interacts with the payment API, only the integration or E2E test will catch it. The key is matching the test level to the type of risk.