I went to an aquarium store and saw three amazing fish.
1. Genetically mutated yellow tang that is white but not albino. It is considered priceless given its the only know one.
2. Jewel tang. Price of 2,999.99 USD. It is only found around 3 islands in the world.
3. Zebra Tang. Price 4,999.99 USD. It is one of two ever imported to the U.S. It does have a sickness and may die.
Common names came sometimes be tricky as I was trying to look up which fish these might be. I'm assuming the Jewel Tang is a Z. gemmatum? If so, they do have a limited area of collection so that raises the price.
As for the White tang, besides my other post about mine, there is also a pic of a partially white "yellow" tang in the book "Dr. Burgess's Atlas of Marine Aquarium Fishes, plate #506. While rare, this white yellow tang is not unique as I can now say I've seen 2 ( and owned 1 of them) beside this one you describe. Buyer beware.
Assuming, again, the Zebra tang is A. polyzona, again, because of where it comes from is what makes it so expensive as well as a rarity to the trade. It may very well be common in it's natural habitat. It might also be a mutation of the convict tang which has been localized to the far corners of the seas. With so many mutations coming into the trade, especially in Tangs, ( i.e. white cheeck/ achillies hybrids, powder brown/powder blue hybrids, etc) as well as the mimic tang A. pyroferus, I'd like to see a few more before spending that kind of money on this fish.
Keep in mind that what's rare today may be old hat tomorrow. I was blessed to be shipped one of the first 2 C. declevis Butterflies that entered the U.S. back in the 1980s. ( The Wiakiki Aquarium got the other one.) It had a price tag of $1,000 on it ( and that was in the 1980s.) We got it on our local news program as well as it was the buzz in the rival aquarium stores. It was purchased by a foreign hobbyist visiting the U.S. Today, you can pick that fish up almost daily and for a lot less money. Sometimes, patience pays
Hope this enlightens