Honestly, it's the combination of being a collector's item, and they were made very well, and it's quite likely that it's been refaced by a master refacer at that price which is worth a lot too. If you don't care about the collector's item part, you can get just as good a mouthpiece from a modern link copy, but if you want it just as good, expect to pay what is still serious dough ($400-1000 probably). Modern copies that I would put in the same category of quality include Theo Wanne, Aaron Drake, Navarre, Ted Klum, and Mouthpiece Cafe, among others. Very small physical differences make a big playing difference in mouthpieces, to the point that the skill of the finisher is significant. When you find the right high end mouthpiece for your horn and embouchure, you won't believe how well it plays and $500-1000 is totally worth it.