The R & R of the transmission is a 6 hour job. The proper reseal requires the removal of the front of the tranny to access the front pump from the rear. This is necessary to replace the large O-ring that surrounds the steel front pump and seals it to the aluminum case. This is probably the leak as often as not.
This is an easy two more hours work. Skilled transbuilders usually get 8-10 hours to do this and the seals are cheap. Labor at Ca prices will have that part of the bill higher than $350.
I can only say that someone doing the job for 350 is either doing it in their backyard or is living in poverty. Not a welcome reference to his skills in my opinion.
Bosch Master, ASE Master, L1
33 years MB technician