I got similar experience as you. I had transmission problem before for my 85 380SE and 2 shops quoted me $3000 to re-build the transmission.
The transmission was having the problem of shifting.
I didn't believe them so I bought a new filter, a gasket and replaced the fluid and the filter myself. After that, the transmission problem is fixed and my car has been running for 1 year without any transmission problems.
The whole cost was $31.
You need to find a reputable shop. Take care.