In my careers, the most expensive conventional tool would probably be a multi-channel oscilloscope w/ logic analyzer. It wasn't as quick to setup and use as an (also expensive) digital multi-meter, but over the years I used that thing to solve more problems (with hardware and software) with electronics than I can recall. It's been years since I needed it however, and it would only help me with a small number of the current problems I have to tackle. I often yearn for the days when all I had to do was figure out which signals to watch and then decode them in front of others so they could witness learning through observation for themselves.