Compare a repair cost with the number of additional useful years you reasonably expect it to provide.
Repair Cost per Added Year Calculator
Formula
How to use the result
The difficult input is expected added life. Use service history and qualified assessment rather than treating the calculator output as a prediction.
What the calculator does not know
This site does not diagnose faults or provide repair instructions. Stop using items that may be unsafe, recalled, electrically hazardous, fire-damaged, structurally compromised, leaking dangerous substances or otherwise safety-critical, and follow manufacturer, recall-authority and qualified service-professional guidance.
Improve the estimate
Keep the inputs used for the decision and compare the estimate with actual results. A simple calculator is most useful when it makes the assumptions visible enough to learn from the difference.