If your looking to make a profit, then no, 20K is not a great price. BUT, if your looking for a Met pro for a great price after selling the other three, I would go for it. As others have said, try to negotiate a better price but do not low ball. If the guy is firm, you won't get hurt even at 20K. Prices are going up, up, up. Plus you get the experience of cleaning and fixing up newer Sterns. I see you don't have any moderns in your collection.
Bottom line: Try to get a better price. If he's firm, pay the 20K.