I'd expect to pay $800 for a fair to good condition NAPCO
kit. If it was attached to a truck and the truck was essentially fodder for the scrap man (in my eyes) then I'd be inclined to deduct from that (cost of gas for torch + time to hack and haul, etc.).
Not to scare you, but the NAPCO axles have the potential to be an expensive rebuild -- a sloppy hub could mean serious $$$ outlay.
http://www.napcopowrpak.com/
http://autos.groups.yahoo.com/group/napco4wd/
Good luck!