In particular, I think that the "koi" scopas trend has gotten out of hand. When I got in the hobby only a few years ago, they were tough to find, and always ranged at around 600$, basically like gem tangs. Usually the specimens offered were pretty nice, with some whites and blues as well. They're super easy to find now, most sites that sell any unique fish have loads of them. One site in particular gets in a few dozen every couple of weeks. Most of these look pretty bad to me, mainly brown with patches of yellow here and there. I prefer the regular scopas, these ones look almost diseased to me, even though they're technically healthy. And still, despite the massive availability and the somewhat low quality fish, these are now all over 2000$! A bunch of them are even higher, too. Here's a "tri-color koi" scopas that's for sale right now at 6000$! I count two real colors, a few discolored patches, and one ugly fish!
Compare that to a "normal" scopas tang. Can you guess which one I'd rather have? The "normal", by the way, costs 50$. If you want to buy a "koi" tang, go for it, but that's perhaps not the soundest decision the way I see it.
When it comes to the gem tangs, the "designer" ones I see are either normal specimens with slightly variable spots, or seriously sick with HLLE. Given that they cost only 200$ on a wholesaler's list, they're overpriced already. They aren't actually very rare in the wild, they just live alone and closer to the rock than other tangs, making them hard to catch. "Hard to catch" shouldn't make a fish $800. I'd imagine that engineer gobies and blennies aren't a walk in the park to get in a net, but they cost 40$. And clown tangs, giant, elusive, solitary, fast, and colorful fish as they are, cost less than $100.
Long story short? Most "rare" Zebrasoma tangs are overpriced for no particular reason. Gems aren't that uncommon, and koi scopas tangs look like moldy bread. Vendors raise prices beyond where they should be, and we just accept that these fish are "rare" or "exclusive" without looking into it. Basically, buy a regular scopas tang, take good care of it, and you'll have a fish that outshines any "koi" tang.