I don't think they're very rare. I've been to 3 different shops in my area that have all had platinum angels and they were selling them for $5-$6 each for 2-3 inch ones.
The thing with angels is that they're so easy to breed. It's a lot more common to find a platinum angel than a platinum datnoid or barramundi because those fish can't breed or are very hard to breed at farms.