It averages out at under £1300 per outfit. It’s part of her role to look well-presented and appropriately dressed for whatever the occasion is. The events she attends are also attended by people who expect the Princess of Wales to be well turned out so that figure seems okay.
Btw she’s not “Princess Kate”. She’s Catherine, Duchess of Cambridge, or the Princess of Wales. She’s “Kate” in the press as a hangover from her single days.