660 and 650 tractors can sell anywhere from $2800 on up to $4500 depending on how popular they are in your area, mechanical condition, appearance, implements included, etc etc.
I just talked to a man in Kansas who is selling an 861 (live pto, bigger engine, a couple years newer) for $3,000. Claims it runs great and looks really good. Price includes a loader that is not installed. Drawback to the loader is that the tractor does not have p/s (main reason it was removed). Also, the tractor was originally a LP version that has been converted to gasoline. LP tank is still there, though. I'm waiting for him to send some pics.
I bought my 850 (the one in the pic) for $3000. Price included an old hay rake that fell apart 2 years later, a loader (Wagner) that I sold for $400, and a Dearborn sickle mower that was missing some parts (sold for $50). Since then, I've added lights (headlights, worklight, taillight), bumper, toolbox, new wiring, glass jar cleaner (not in pic), and front wheel weights.