I did my estimation more on royal level, serious cash is the sale of diadems, suites, etc. This is a small brooch. When I would be in need for serious cash and enforced to sell family heirlooms, I probably would go for larger pieces generating serious cash in once.
But you are right, the sapphire is not that small and maybe the possible royal provenance will give a boost to the bids to purchase the item. I believe it is not for sure that this item was actually sold by the Kents?