Kate not only wears 'affordable' luxury, she often wears high street brands - Topshop, Zara, Reiss etc. In fact she's been criticised by the fashion establishment in this country for wearing high street brands too much.
It seems to me that she's more or less striking the right balance. If some people are saying she wears too much designer gear and others are saying she's wearing too much high street fashion, that tells me that she's just about getting it right.
I have no doubt that this dress will be re-worn several times. Kate has already shown that she has no issue in recycling outfits (and gets criticised by posters on this forum when she does so). No-one comments on the fact that William was probably wearing an ensemble that cost upwards of £4-5k. Prince Charles has been known to favour hand made Jermyn St shoes that cost £3k per pair. But they're worn many times over many years and work out as a good deal. I have no doubt the same will be the case for this Mulberry dress.