You just have to stay out of those antique shops! If you're looking for a good bargain, try a repair shop. I almost bought a treadle Singer in one, and it would have cost me $100 including a beautiful cabinet. Plus it was in perfect working order, and had a ton of attachments (1928 model). The only problem was that the shipping to get it home was over $400. So I passed.
Other good places to check are Craig's list, yard sales, estate sales, and just getting the word out that you're looking for one. Eventually, one will turn up. Make sure it works and has attachments before you consider it a bargain. Happy Hunting!