Try ebay using the completed items function to see what people have paid recently.
Another couple of sites are
http://www.pacificrimcamera.com which usually has some for sale and
http://www.ritzcam.com.
A quick look showed me that the Prominet 1 with the "cheap" lens tend to run about $100 - $200 depending on the shape of the camera and how patient you are. Manuals add a lot to the value. The faster lenses are worth a lot more (they can be used in Leicas).
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I have some Voigthlanders in my collection but mostly collect turn of century to WWII era. Voightlander has always had a special place in my heart since it was my first pro level camera.
The Prominent 1/2 are great cameras for the collection but not very friendly to shoot (horrible ergonomics).