The problem with historic pieces with a huge price tag is that if "the royal family" buys them, they look as if they are taking food/medical/defense/etc care away from others - because everyone believes it comes out of public funding.
Were I a years gone royal, I'd have put it all in the crown ownership and loaned it. But years ago, no one had any idea (or much idea) that people would hate the idea of royals buying back tiaras. Sad for us, practical for the BRF.
QUOTE=Iluvbertie;1920280]Obviously the BRF don't think this piece is all that important or they would have probably bought it from the Harewood's.
It was a personal gift to her daughter by Queen Mary who would have known that it would not be protected by the sovereign to sovereign tax free arrangements and knew about death duties as they had come into being while her husband was Prince of Wales. She still decided to give it permanently to her daughter.
If it was returned to the monarchy it would probably end up in a vault somewhere never to be seen, or even broken up to create new pieces for someone - which is where the Queen's bracelet came from along with her engagement ring.