that seems awful steep of a price. Blacksmith bellows probably have had a rough life to begin with not to mention few were probably saved in any shape, But as antiques go on the market i couldn't estimate the actual worth of one as a collectable. If its been hanging there for any length of time i would ask about it, give it a real good going over like your an expert, take notes and such lol then give a fair offer for it. Once all the damage has been pointed out they may drop their price.
As for rebuilding one now that would be a fun project to jump on. From what little i have read most of the metal parts can be hand made so no real problem replacing those.