Here are the 1st three 550's I pull up on Autotrader. Asking prices range $33k-$39k:
http://tinyurl.com/ye92b7h
http://tinyurl.com/yjrahyz
http://tinyurl.com/y8nswje
I'm in no way trying to justify that this is the proper market value, but the guy the OP is dealing with isn't terribly out of line with what these sellers are asking for. I bet if the OP keeps looking though he/she can find one in the $22-$26k range too.