M90s from a Supercoupe should normally sell for around $100 to $150 for a working condition early style blower, and maybe up to $250 for a latter style. The early style have a oval shaped inlet in the rear and the later style have a square or rectangular port and flow alot better. Also, the early style have a slightly smaller pulley than a later style and spin just a bit faster.
Might be a good idea to try to see what style blowers he has that way you can try to get him to come down on the price if they are all the early style. Early style blowers were used from 89-93 and 94-95 had the later style.