If you're a princess, you can splurge on a few heirloom quality pieces for galas and important events. And maybe also quality, classic pieces you would use often.
But most accessories for day-to-day wear need not be super expensive and so high quality. They just have to look good. Costume jewelry would make sense because you can have many to mix and match with different outfits and occasions. Not super expensive so you can collect a lot of them. See Maxima who wears a ton of eclectic costume jewelry to complement her many outfits. Max really has fun with her accessories. Doesn't make sense to spend a ton of money on these.
I don't understand spending $1600 on pretty basic gold earrings when she has so many earrings of similar quality.
Jackie Kennedy wore an iconic pearl necklace. It was faux-pearl. When it went up for auction after her death, The Franklin Mint bought it for $$$ then sold reproductions. https://www.worthpoint.co...jackies-pearls-1728836572
Royal jewelry need not be super expensive. What matters is the taste of the wearer.