I'd offer $150 at first and go up from there, if you think it's worth it. You should probably check the bike out in person before making any offer to see if it fits and if it's actually in the condition described in the ad.
__________________
.
Reason is a scoundrel, stupidity is direct and honest. –Ivan Fyodorovich Karamazov
Man's mind, once stretched by a new idea, never regains its original dimensions. –Oliver Wendell Holmes