A good rule for SS pinballs is anything under a thousand dollars has the potential of being a good deal. If it's a DMD, even more so. But condition is king. It could be a Medieval Madness but if the playfield was missing all of the paint, the cabinet was beat up, and the boards were shot, it wouldn't be worth much.
If you're capable of making an old machine look good again, you have the ability to make a good deal out of nothing.