Prices can be all over the map. Are you willing to work on it yourself? Some times the best looking machines have something wrong. Check anything you buy real good - pay attention to the details. Take plenty of time to look it over. Many vintage machines will sew rings around the new stuff. The more different things the machine can do the more you need to look over. Does it turn? Does it make a good stitch? Is the tension set right? Rubber parts melted or hard? Does the clutch disengage? Is the machine turning easy or is it stiff? What does the motor sound like? How fast does it turn? If you are buying a project you should not have to spend much. If you are getting one that is in good working order at least you won't have to take it somewhere to get it worked on or work on it yourself.
I had an embarrassing moment last night. I was showing a young man two different Singer 401s. One was cold and the other was room temp. The cold machine just did not want to turn. I think the oil must have gotten too cold and wanted to set up. The motor ran free when I disengaged the clutch. That tells me the motor is fine but the machine needs to be cleaned some more and re-oiled... maybe just kept a bit warmer.