Depends on condition. I paid $125 for a rough looking machine about 10 yrs ago in Angleton Tex at an antique store. She sews well, but the decals are faded, there are some other condition issues. I would maybe pay $300 for one in almost perfect condition, with a pretty good case, all the extra attachments (ruffler, hemmer, wide hemmer, tucker & buttonholer maybe, edger), an original book.
For a special machine like maybe a Centennial in above condition, I'd maybe go $500. A 222 would go for $800-900.
JMO on prices, I still see an awful lot of folks that get 'em at yard or estate sales for under $50! That's my dream, to find one in really nice condition for under $100 ;)