Since they're out of production it's pretty much dependant on what the customer wants to pay. I think they listed above $200 and you could find them at internet dealers as low as $155. I got mine when the last of them were pretty near gone for around $180. This price range starts to compete with custom knives which puts a ceiling on the top end.
I would guess that somewhere between $150 and $250 would be possible, depending on buyer personal interest. The best place to find a price would be the Buck forum here at Bladeforums or ebay. If you search "closed" auctions on ebay for Buck and BG42, then for Buck and BG-42, then for Buck and Series you may find what some have sold for.