how much to ask for X heads?

Decent bare castings are worth 100.00 Usable used heads are 300 easy.
The difference is in their condition. At 40 years old the majority of those heads heve been reworked before and have good used valves and hardened seats.
If yours are all original and the valves have recieded into the heads and worn razor sharp on the edges, you may as well call them bare heads.