Unfortunately there are not sites with that type of information. But the value of these cymbals depends greatly on size. You'll find 14" and 15" quite often and these don't sell for all that much. But one you start getting into larger sizes, the value shoots up astronomically. Just to give you an idea of their relative rarity, I have never see a pair of 18's, 19's, 20's or 22's offered up for sale on eBay in the last 10n years. But like I said you can see theses cymbal in the smaller sizes fairly frequently.
This is a surprise. Who is looking for these larger K constantinople's? They are quite thick, so wouldn't have the 'bop' appeal of the large K istanbul's. And they're too big for hats. And there aren't that many symphony orchestras looking for vintage cymbals (are there?) So who is looking to pay the big bucks?