Enough so you can afford to do it again should something goes wrong ?
This is the very reason you pay through the nose at shops. The tranny builders I know charge three times the actually cost of the rebuild, some even more. When they get it right the first time, lots of money from your pocket to theirs. Many buy remanufactured units with warranties, so if something goes wrong, the only cost is fluids and labor to the shop. Once again, the customer loses. Most engine builders (shops) do the same.
The waiver idea is the best if you are trying to be fair with pricing. But have it notarized.
