I’m definitely not a coral expert at this point, but looking online it seems prices on these corals vary wildly (I’ve seen it range from $145 for an all blue, 4” specimen up to a ridiculous $4,500 for an “ultra rainbow” kind of specimen). That said, standard pricing for nice pieces of these seems to be from ~$500-$700. The only other Acanthophyllia I’ve seen with this specific color combo was listed as 4-5” for $600.
So, given that it’s a good size and that the color combo seems to be pretty rare, I’d guess $700 is a fair price.