I don't think we can really compare Victoria Beckham to regular people - even posh regular people. She's an entertainer, model, fashion designer and celebrity. If she even does pop into a Tesco, the normal rules don't apply for her. She lives in her own world.
I do find the word "expensive" sort of funny. It works for what we're discussing but the idea of someone saying "You look expensive" puzzles me. Does it mean, "You look the opposite of a cheap date?" "You look high maintenance?" "You look like a lot of money has been spent on the the things smeared and draped over your body?"
I do think there are different ways to interpret it. The OP could have said, "I want to look rich" but that may have pushed us into entirely different directions (old money vs new money, etc.) with people talking about the really, really rich people they know who go about covered in manure and animal hair.
I do think most of us are talking about the same things, though. The sort of people who just look really good, as if they have wealthy, easy lives. The ones with the slim legs and glossy hair and high-quality, well-fitting clothes.
It's decades out of date, now, but it is still worth reading (although American): Dress for Success for Women. The guy who wrote it was an image consultant who did a great deal of research into how people should dress for their careers. And I mean proper research: showing panels of people photos of women in various outfits and saying "Who is the boss? Who is the secretary? Who would you hire?" He learned a LOT about how people perceive intelligence, authority, trustworthiness based on clothes and grooming. He discusses the pitfalls of such things as "casual Fridays" (undermines women executives' authority), how to command respect in various fields and regions, how to dress for the job you want to be promoted to. Even how to attract a rich mate. It's absolutely fascinating.
I remember his discussing taking ambitious women from humble backgrounds and schooling them in class and quality. He'd literally send them to the most expensive stores to touch and try on clothes until they could see the difference. Then, with that knowledge under their belts, they could seek out versions they could afford, with careful attention to fit and colour. It's from the late 80's, I think, but the main principles still apply.