I picked up one about a year ago for my '95 R100RT from an '82 R100 for $300 and after one fall/winter season it's been fine. I sent my transmission to Boxers by Bruce (in Pantego, TX) and had it rebuilt for a little over $300. That was all gaskets, bearings, seals, pawl spring, and new neutral switch. I now have it "on the shelf" as a spare when one of the others bites the dust.
I was lucky in that I was able to be patient and got it for a good price...