The British Royal Family differ from other royal houses in that they are not allowed to accept free clothing nor are they allowed to receive items at a discount (although I believe they do receive cars etc. at a discount so how much they follow that rule is unknown). I highly doubt MM has paid for anything out of her pocket since the engagement was announced. She wasn't wearing Givenchy or 40k dresses before her engagement (beisdes that which she borrowed for red carpet events) which tells me that she couldn't afford it before getting married.
Prince Harry is very wealthy, but very very little of that incredible wealth has come from a private job (his work in the military doesn't afford him 40k dreses). Even the money left to him from his mother started as taxpayer money (as she was wealthy because of her divorce settlement. My understanding is that her own family were not personally funding her or helping her out once she was married). The idea that MM borrowed her 40K engagement dress is beside the point because the mere fact that she wore it sends a very bad message to the British public.
The royal family are wealthy because their status as representives and heads of state have allowed them ownership of property, land, businesses (like the Duchy), priceless artwork and jewerly. Again, all of this has trickled up to them because of their position and because of taxpayer money.
In my own viewpoint, I am not able to excuse away the extreme cost of the clothing worn by MM or KM because the only reason they are able to afford their lifestyle is because of the British tax payer (now and historically). I agree with the poster who said that MM is like a kid with a credit card. Her spending is insane and it can't be excused by the idea that she has to build a wardrobe. She doesn't need a wardrobe that costs hundreds of thousands of pounds for her first year of "work". I have had these same thoughts regarding KM. I wouldn't care as much if they were only spending large amounts of money on ball gowns for state dinners because those are few and far between, but when you tally up how much MM and KM are spending on basic everyday "work" clothing the cost is astounding.