Associational pricing is all the rage in the book world. Now that every book can be found online, the only way to get premium prices is to offer one-of-a-kind pieces, things that nobody else can offer.
By itself, a set of the Foundation Trilogy is about $6000. Exceptional condition might push it up higher. You need Asimov’s signature, though, to to get into five figures. That can add $1-3000 to the price of a first edition.
I think the Norton books are overpriced at $7500 each, but if the right collector comes around they’ll get pounced upon.