Saw a used GK 700RB today in the local music store for $350...looked clean. Didn't play it was just kind of browsing. Looking at possibly getting rid of my SVTH-350 and upgrading within the next month or so.
700RB MarkII seems to go anywhere from $300 to $400 on the GC used website, depending on condition and location. So your price is right in that range. I've never seen one much cheaper that was in good working condition.