Loving this thread!
I saw a picture of Princess Charlene of Monaco the other day and thought she looked "expensive". Note the neutral clothes-and she always seems to wear expensive designer clothes. Even though she doesn't have long "swishy" hair, she looks very chic with her pixie cut. I think she looks more expensive than the Duchess of Cambridge, although I do love Kate!! 
This blog is great and explains it so well:-
howtobechic.blogspot.co.uk/2013/09/how-to-look-expensive.html
IMO, most important for this look is:-
Being clean and showered
Wearing fresh, ironed clothes
Manicured nails-short-NOT talons with bits of glitter, designs etc. Can be painted but classic colours, or buffed
No cheap jewellery piled on, instead classic items, a good watch and only rings should be wedding, engagemant, eternity and maybe one other on other hand
No towering heels you can barely walk in
No muffin tops hanging over too tight jeans
Any tattoos should always be somewhere where no-one can see them
Subtle make-up, no caterpillar eyebrows or God forbid, Kim Kardashian facial contouring)
As far as clothing goes, I hate to say it, but I do think very expensive clothes DO look expensive, although you can mix expensive with the odd cheaper item. Neutral colours, (as mentioned before)-whites, black, camel, navy, odd colour like red or maybe orange or pink to brighten (this could be a bag, scarf or roll neck jumper, maybe a lipstick)
Bags, coats, shoes (if poss) should always be good quality, the best you can afford.
I do think white teeth help with this look (NOT Rylan Clark/Simon Cowell white), glowing skin, glossy hair (but not overly long WAG hair extensions).
If only I could follow my own advice