If you are serious about selling, you will probably have to put a price on it. If you just want to find the value, list it on Ebay with a $1 starting bid and a $20k reserve. That way you don't have to commit to selling, but might get an idea of true market value for the price of the listing fee. Kind of a sleezy thing to do, but Ebay seems full of sleezy people lately so you might as well play the same game.
Given the problems current owners seem to be having, 1/2 of the list price is probably as much as you could expect from a knowledgeable buyer here.