Excellent reply JL! The only thing I might add is that there are 3 basic types of these skins:
Big Crocs from Australia/New Guinea and Africa (Porosus and Niloticus)
South American Crocs (Caiman, Panamundo, etc.)
The most desirable by far is Porosus. This is what Hermes pretty much has the world supply of (American Tanning in Georgia also gets some of these skins, FWIU) and is what the Grace Kelly bags are made of. You can usually identify Porosus by a small dot on each scale. Also, the transition from belly to flanks is gradual on this skin and therefore you have a very nice cutting. There is a picture of a Chelsea boot done in Porosus, I believe, on Centipedes' link to G&G's special order shoes. Niloticus and American Alligator are similiar, but Niloticus is usually much larger, so the scales are, obviously, larger. American Gator is usually available in anywhere from 15-40cm skins while Niloticus can be available in 50+ cm skins. The Hermes agent for Italy holds the rights from the largest supplier of raw skins in Niloticus of smaller sizes, and this is coming on the market now and is virtually indistinguishable from American Alligator. Many of the better Italian shoe factories are now using this as American Alligator is much less dependable a resource. As JL points out, the most abundant, and the most varied as far as quality is concerned, is Caiman. The only suitable portion of these skins for 'better' work is the flanks, and they can be both beautiful and horrible.
around $12,000 retail
around $8,000 retail (although American Alligator products can swing wildly in price, as there are 5 grades available...Grade 1 being almost 4x more than Grade 5)
Finally, this market is priced by the centimeter, and the raw cost has risen more than 40% over the last 18 months. There are, for all intents and purposes, 4 tanneries left in the world that supply any given manufacturer.