Whatever someone will pay.
If you're referring to the 60's tube Coliseum head, they are pretty scarce; there really
isn't much of a track record of sales to go by. I'd say that if you could find one for sale it depends on the seller, and if they think they have a collectable "rare gem" or just an outdated "old amp".
I think I remember that someone on this forum mentioned having one, or buying one a couple of months ago, but I don't remember any transaction pricing being discussed. Try a search.
I would certainly like to have one myself!