I don't think I've ever come across knitwear in a charity shop with a price tag more than £5 - except Traid, has anyone else noticed their prices have sky rocketed over that last year or so? You do have to hunt around a bit more to find something decent but there is nothing like that warm fuzzy excited feeling you get when you find something perfect, totally unique and that you'll get change from a tenner. Not forgetting that you have the knowledge that you're giving to charity and being green by recycling. I have a really good relationship with my local charity stores, I shop there regularly and I always donate good quality clothing whenever I have a huge wardrobe clear out.
So before you go into that nice fancy vintage store don't forget to check out the charity shops the cheaper end of town, you never know what you could find.