Stewart,
Basically, you've got a reasonably hard to find collectible, which aficionados will pay more for based on condition and paperwork. For one in superb condition, $200-250 USD would not be unreasonable to expect in a bidding situation. If I didn't already own one in beautiful condition that's what I might be willing to pay. Incidentally, a near-mint standard 804 Audubon will fetch about $200 nowadays, and an 804ED about $400 if you can find one with nice cosmetics and in working condition. I think the 826 Audubon/Kestrel is less plentiful than the standard 804, but I have no production or sales numbers to base that on. Other than not being waterproof, I concur that both the 8.5x44 and 10x50 Audubons are magnificent instruments, only surpassed by the 804ED, in my opinion.
Incidentally, what are the first two digits of the serial number, i.e., the date, and is "Audubon" mentioned on it?
Ed