Ah yes, the golden "masked" butterfly..I had a lengthy discussion with Robert Fenner about these butterflies, according to him they are hardy butteflies, will nip at corals, but I tend to feed my fish 3-4X a day. With that said, I have tried 2 times to add them to my tank and both times they ending up dying and I cannot tell you why that is..
Yes, they are pricey as they are only found in the Red Sea.
I would highly suggest that you QT them for at least 4-6 wks before adding them to your tank and where ever you are getting them from make sure that they are dewormed..if they can't tell you, assume they are NOT, and do a 5-7 day PraziPro treatment while they are in QT.