Well, although discontinued, they still sell for ~ $420 shipped. Unfortunately lead acid batteries often don't fare well in long-term non-use storage, and the specialized replacement battery pack is ~ $180. If I had to replace the battery pack I wouldn't want to pay more than $95 shipped so that my total cost was no more than 65% of new.
If the battery pack was in excellent condition with the majority of its recharge cycles left (and I'm not sure how to know or test for that), I would say ~ $225 shipped.