Late to the thread, but I've been thinking about this and need to make a Marmite-sort of post here it will mention House of Colour.
I agree with what's been said re grooming, taking care of clothes, spending money, (often) being slim and tallish, etc, but I also believe that it's only possible to actually achieve the specific look that a lot of people on this thread seem to be referring to, if you have a specific style personality. (By style personality I mean the categories discussed here - NOT a HoC link
).
Not everyone looks "right" or their best in blazers, loafers, silk scarves, smart jeans, wool coats, riding boots or whatever item you want to ascribe to the "expensive" looking woman. As has been said, these items usually belong to a very classic look, something which I believe you need to have a classic style personality to carry off.
I'll use my Dsis as an example. She has been diagnosed as an Ingenue Natural by HoC (basically relaxed and feminine). If she tried to dress like a preppy Ralph Lauren model she would look ridiculous (and she's a curly girl so couldn't do the sleek highlighted hair either). To look her most "expensive" she would need to wear items according to her style personality, because by definition those items would make her look most like "her".
TL;DR - most people seem to have a specific look in their mind which they associate with looking expensive, and I think that people who don't suit this look can never look as expensive in that way.
The exception would be the woman mentioned above wearing the suede skirt and fur gilet; I would assume that because 1) these items suit her style personality, so she can pull them off, and 2) these items are currently in fashion, and 3) these items may be made of very expensive materials, she can still look expensive without looking classic.