You do generally get what you pay for, although there are some good cheaper alternatives. The Sigma HD brushes are really good. I would recommend a stippling brush rather than flat foundation brush, as the result is usually better. Samantha Chapman's Real Technique brushes are good too.
Pixiwoo fan, Mippy? I hate all foundation brushes/putting on foundation with a brush, iykwim. So I'd say try a cheapie one of a few varieties first, such as a buffing brush, bigger stippling/flat ended brush, wedges... Then you can see which type of brush you prefer using for foundation without buying an expensive one, and then buy a more expensive one of your favourite. long winded much?