I'm just thinking about the reverse engineering that went on re the merching of the dress. I don't believe MM will have got actual cash in her hands. I think this is about public image for her (photographed looking glossy and well dressed on front pages around the world --> more designers wanting to dress her for free back home) and worldwide publicity for the designer.
So MM's team would have had to identify a designer who makes suitable clothes; strike a deal that works for both parties; organize somewhere suitable MM could be photographed wearing those clothes, extra points if it makes Harry look good too; find a photographer; make sure that photographer is the only one issuing photographs for public consumption; get unsuitable photos scrubbed; get photos sent to MM's team immediately for editing and distribution; photos are sent to designer and People magazine with instructions; they upload onto their websites asap.
No wonder the actual hospital visit appeared like a distraction: to the team, who are no doubt working in a culture where everything and everyone is geared towards maximising MM and what MM wants at pain of real-time bullying, it absolutely is. It's like 5% of the actual point of the whole thing, which is merching.
When you think of all the consents and exclusives and paperwork and logistics, it's quite a big undertaking. That's for one outfit. Same thing all over again for the next one. It's not a surprise she's only using one designer, the work involved striking these deals with different designers and making sure none gets a better deal than the other would be too much.
I don't think MM actually cares about clothes that much tbh: she's all about the money. She'll sell clothes, make up, jam, wine, honey, her appearances, a podcast, an interview, the back of her children's heads - anything as long as it makes her money or leads to her making money. So it would have been lovely wearing free Dior and Givenchy but it wouldn't have been enough - she will have felt that they got free publicity from her and raised their profiles by dressing her, she got nothing out of it because she got no money, the only currency she deals in.