Prices vary widely. What seems reasonable in one area may be high or low in another. Ebay gives a good idea of what top dollar is on machines. Watch your local Craig's List. After a while you'll get a feel for what's a reasonable asking price in your area. You'll start seeing what the range of asking prices is and what ads hang around forever. Most of my cheapest machines are thrift store finds.
As KalamaQuilts mentioned we do like to brag about getting machines for cheap but that's not the whole story. My own experience is I'm a bottom feeder. I buy cheap machines. Generally there's good reasons why they're cheap. Something on them needs to be fixed for them to work and the cosmetics aren't that great on most of them. You get what you pay for. A complete machine in good cosmetic condition is worth more than a beater that's missing parts, same as anything else.
Rodney