I've always been into classic clothes and they get along with me, as well. I really adore Kate Middleton's look. It seems that better fabrics look good in these classic clothes, though. I've tried buying some upper-mid scale brands (Banana Republic, JCrew) on e-bay, but it's hard to get fit right without trying them on.
Banana Republic I have mixed feeling about. I buy stuff from there but its really not -much- better than the Gap (its parent company). I consider it more staples then 'upper mid'. Be aware that the Gap/Banana Republic have 30% sales monthly or more, and you should probably never buy anything there if its not a 25% off or better sale day.
Spend your money on simple core pieces you can get a lot of use out of.
Keep an eye out for brooks brothers at deep discount. All women's clothing there was 40% off a couple days ago - and I suspect will be again post Christmas.
A lot of Kate Midletons clothes are inexpensive. They come from UK high strret stores like River Island, Topshop, Awear. That's why she's become such an icon
Buying clothes online can be dodgy as sizes are not universal. Measure yourself correctly, look at the sites sizing guide, some retailers especially on ebay and amazon tell you to go by their size guide not the sites. Buy one inexpensive thing from them and see how it fits and go from there