I am glued to this thread. It's brilliant. I bet the woman in Sainbury's who inspired it has no idea how many women she has influenced.
I agree with a lot of the advice on here, but one thing that really helps when trying to achieve the 'expensive' look is time. Lots of it! Before the DCs I was far more chic/'put together'/expensive/groomed (whatever you want to call it) because I had more time to go shopping, try things on and take items back to shops if they weren't right.
These days, because my three DCs are none too keen on being dragged around shopping centres and department stores, I do all my shopping online. I panic buy for occasions, I purchase things which look good on models but look pants on me and - worst of all - I don't bother sending things back if they look 'not quite right' because I can't be arsed. I think 'oh that'll do'... My bar has been set a lot lower.
Trying things on in different sizes, I think, is crucial in getting a good fit, which in turn makes you look more 'expensive'.
In answer to the OP's original question - I actually think you can find expensive-looking clothes in high street shops. You just have to rummage. H&M is a bit like a jumble sale in that there's lots of 'cheap'-looking garments but, over the years, I have found some real gems. There was a Breton long-sleeved t-shirt last season which was made with a thick sweatshirt-type material (sorry, not up on my fabrics) and it feels and looks luxe, in my opinion.
The problem is, however, to find these treasures, you need to either be very lucky, or spend a long time searching.