The transmission in the M5 is basically a sealed unit. You are basically looking at a replacement. The Dealer will want approx $3.5-4.5K for a new(?) one. I just replaced mine (shifting into 3rd made/felt an awful crunching noise) with a rebuilt unit which was in the order of $2400. If you buy from the dealer make sure you get it from someone offering the BMW CCA discount.

I heard this is a somewhat common problem. The car (1991 Us Spec 80K miles) is fine in every gear but fourth. In fourth, it will slip into neutral as soon as you release the gas pedal. It also seems very difficult to get it into fourth unless you rev match/double clutch on down shifts.

Any ideas on problem/solution and typical cost...should I get a rebuilt/used tranny instead?

Thanks,

-Adam-

There is no magic here.
The Transmission can be disassembled with the correct knowledge and tools.
My M6 used to jump out of 1st gear. It also has the Getrag 280 gear box. I located a BMW service bulletin that called for the replacement of the detent spring assessed on the side of the transmission. About a couple of hour job and it hasen't come out of gear since. I would inspect the detent springs just incase one has failed.