I'm not sure client's time wasted is the right way to measure. For example, I would prioritize a high-risk security related bug (regardless of its effect on my systems performance) over a memory leak.
I never stated that client time was the thing to measure. I'm saying (now repeating) that PRESENT COST is a better basis than FUTURE TIME GUESS.
The whole problem is management wants to maximize dev-time so they try to cram bugs to make your time full - based on a guess. It's ass backwards.
Measure the problem then budget resources to fix.