I just re-read your post, and you said "if" he wouldn't budge. I thought you said that he wouldn't budge.

I'd offer something like $300 or $350, and try to keep it in the $3xx range, if possible, with $400 as my probable max. I would also look it over very carefully to see what shape it's in, and try to determine how much actual use it got. Little things like tires, pedals, etc... mean a lot at this price range. If you like and want the ones on the bike, that makes a big difference in what it's worth. If you don't, those things cost money. Seats are always guesswork, unless you've ridden that particular seat before.