1971 Buik Skylark

Hey guys, I hope this doesn't go against the selling rule....but my brother dives my Grandmas 1971 Buik skylark....he was thinking of selling. What should he ask for it?
I am just doing him a favor.
It is a 4 door and it is pretty straight! runs great but recently the heater quit working.
It has a hard top as well. other than that I don't know what else to post.
Thanks guys!