Less expensive to who? Your reverse clutch came apart most likely. A simple straight forward affair that probably should include doing all the seals and other clutchs, but if he installed a tranny he had in stock I doubt you saved by the deal. You would have saved if you had went to a shop capable of fixing the car without shotgun parts replacement. His replacing the trans with a unit for that type of repair tells me that he don't fix them himself.
This isn't a put down of the final product, just the diagnosis. it's just that one will get a product for less money by eliminating the middle man (if that middleman isn't a producer).
Bosch Master, ASE Master, L1
33 years MB technician