There was a thread on the H.A.M.B. a week or so ago about
Grancor heads. The gist of it was that all
Grancor equipment was good. (This is easy to believe because of the company's history.) One guy on that thread said that he picked up a set of pre-49 heads (like those shown) on eBay for $700 and later re-sold them for an amount that made him very happy. If the heads are in good shape, I would expect that you could expect the same results. Have you seen the price of new Offenhauser heads lately (if you can even find them)?
I would also expect the manifolds value would be increased if being sold with the heads as a matched set, rather than by itself.
EDIT : I just checked the thread I referenced, and the guy paid $700 for the heads in 2015.
I think things like this have gone up since then.