They dont make them anymore so they have went up in value a little and I assume will continue over the years. Id say $2500-3500 is a good price for a modded one.
However If I were you unless the seller had all mod documents(porter,machinest) Id rather have one as close to stock as I could get. Id look for the basics. Find one with pipes, intake, filters, head, etc... that you like but with no motor work. Get your motor work done yourself so you know what youre getting.