You need a way to assign technical debt to a process or practice, then you can evaluate the cost of that process over time.

Could be something like... Our team spent 500 hours fixing bugs introduced during overtime, and we spent 250 hours of overtime to introduce those bugs in the first place. So an additional hour of overtime will likely introduce 2 hours of technical debt.

Or maybe... Code written using inheritance hierarchies creates 40 minutes of muda per hour, but using composition only creates 20 minutes. Or... every hour spent on throw-away prototypes reduces muda by 2 hours.