Do some searches for similar kits here and on RC groups in the classifieds along with Ebay completed listings. Kits hold their value fairly well, and a few become collectible and will bring a premium. I generally expect an old kit to bring 70-80% of what a similar current production kit will bring, so do some searching for who else is making something similar. Of course, you could just list it on Ebay and start it at $1. Most of the time, it will sell for what it's really worth as kit builders and hobbyists of all kinds scan the auctions pretty often.