This thread has been a really interesting read!
Like MrsCB, I have no desire to look expensive. I see my clothes as an external representation of my character. I'd much rather someone looked at me in my Charlotte Olympia kitty flats and thought, 'she looks like fun', rather than 'she's spent a lot on those shoes'.
Anyway, my tuppence worth:
I think it's more or less impossible to look 'expensive' if you shop only the bottom end of the high street - while you might find the odd gem in Primark/New look, a lot of it is acrylic tat.
Grooming counts for a LOT. There is a mum at school who spends a bomb on her wardrobe - it's all Marant, McQueen etc. However she is quite overweight. This in and of itself is not an issue, however her clothes don't fit well and she doesn't dress for her shape. She also usually has several inches of dark roots on display and never wears make up. It totally undermines her expensive wardrobe.
I work in quite a staid, corporate environment so I have to dress in a certain way for client events. I find DVF wrap dresses, well tailored shirts (usually uniqlo, of all places, for cotton, equipment for silk), good pencil skirts (VW red label, jaeger, McQueen) and simple, not too high, heels (Jimmy Choo and Chanel are the best I've found), work well for this. However, none of that is exactly cheap (except the uniqlo shirts!)
I think if I was doing it on a smaller budget I'd look at M&S, as they do have some gems.
I've also seen many a nice outfit let down by a cheap, synthetic bag so that is where I would prioritise funds.