No, it's simple supply and demand. If a fish is scarce in the hobby - for whatever reason whether rare in the ocean, regulatory constraints, difficult to catch, etc. - but nobody wants it then the price will account for that. If it's more common in the hobby, and everybody wants one, then the price will account for that too. Are diamond tails actually scarce? $300 is still a considerable amount for a fish, but don't you think if sellers believed they could get $400 they would.
Since you noted Mauritius ..... I had been on the hunt for a zebra tang. As far as I know it is found solely there. Not many of them find their way into the trade so they command a healthy price. Why do not many find their way in? Dunno for sure, perhaps they are uncommon or perhaps it's just expensive to get them here; or both. When they first showed up on DD, they were in the $800 range ..... and they lasted 5 minutes. Guess what, next time they were $1,200. Not just usual to see them stay around for a week or so before selling. Supply and demand ..... and price elasticity.