I would advise joining the homequiltingsystems group at
http://groups.yahoo.com . This is a very active group with lots of information about different machine/frame setups. There is a listing area in their "files" section where people can describe the used systems they have for sale. Also check the websites of machines (such as Gammill, etc.); they often have an area listing used setups for sale. If you are interested in a specific machine, also Google -- e.g., used HQ16 for sale; you can find lots of pricing info that way.