There are a lot of variables in that question. The best thing you can do is start looking at your local Craig's List to build up an idea of what a fair price would be in your area. What I would pay can be a far different amount than what you would pay. I also tend to be a bottomfeeder, I don't have the finances to spend a lot on machines. Around here if I really, really wanted one, say to complete a collection, maybe up to $100 but most likely I would wait for a less expensive one to come along. I just missed a free one last week.
If it's under $50 and is in decent shape you're not going to go wrong.
The cams and feet are a big plus but they're easy to get on Ebay. Maybe add another $20 to $40 for those.
My 2 cents,
Rodney