That is an excellent price for one that size I think. In my area one that size could go for half again to double that price.
A shot of the bottom wont really help ID- you would actally need to break off a piece, bleach it, and have it examined under a microscope to give a more definite ID. Luckily, almost all of the other non-echino corals that it could be have very similar care requirements, so an exact id isnt really needed to give it good care.