Upload & Sell: On
I have a set of Canon FD lenses that I use on my Canon EOS M with a $10 adapter, and used on my Sony NEX 5N and 7 before that.
I didn't go for the fastest, rarest glass. Just the normal 28, 35, and a 35-70 zoom. I invested maybe a bit over $100 and am quite happy to play with them.
ETA: The EOS M itself is a quite affordable camera too. It is an 18 MP APS-C digital. It has an excellent 22 mm STM IS and a very good 18-55 mm STM IS that are each less than $100 new.
You can get an EOS M body with the 22 mm or 18-55 mm for about $250. Add the other lens used for $75, and get the FD adapter for cheap. It is LCD only, but it is a blast to play with for the price. I bought 2 at the beginning of summer, plus an adapter for my regular Canon lenses.