Actually had this problem in PC repair. A lot of times I'd spend hours/days diagnosing an issue just so I could understand it inside and out and have a quick solution for the future. I could almost never bill customers for it since it would've cost hundreds of Euros (easily going into 1000+) for what ended up being a fairly simple fix (the $10.000 to hit a pipe with a hammer phenomenon). I'm just lucky I had enough slack to be able to do in-depth diagnoses, because it saved me fuckloads of time later on.

Then there were the cases with arbitrary random bluescreens that were impossible to diagnose where I'd upfront tell them I could only do the bare minimum because otherwise they might have to pay 500 Euros for a solution that probably wouldn't work.