Hi Kalman,
I already answered this to you privately, but I thought some general comments about the value of books might be useful to other collectors.
most everyone knows the old saw, what is the three most important things about real estate? the answer, location, location, location.
the three most important things about collectible value of any item, including books, is condition, condition , condition. a lot of collectors, in the heat of auction bidding in particular, have either forgotten or ignored this rule. it should always be remembered that a scarce item in poor condition will always be a scarce item in poor condition.
it is difficult to pin down an exact number on a book like Filson Young's as it doesn't turn up that often. it is relatively scarce. but if this particular copy is in good, fair or poor condition, an asking price of $500+ is way, way too much.
if the copy is in really nice condition, I still think a price in the $500+ range is pushing the high end, but not unreasonably so.
hope this helps!
all the best, Michael (TheManInBlack) T