Of all my brushes, I've spent the most on my foundation brushes. Like I said earlier, I'm not spending 100s, but in comparison to my $1-10 brushes, plopping down $30 for one brush can seem high. Since I wear foundation every day I appear in public, it's an important tool.

Clockwise from the left: Lancome Foundation Brush #2; Bare Escentuals Concealer Brush; Giorgio Armani Blender Brush (travel size); Eco Tools Concealer Brush
The Lancome foundation brush gets the most use - anytime I'm using liquid foundation, it's used. I can't tell what's it's made from, but I"m 99% sure it's synthetic. I should wash my foundation brush more often than I do, which is about 1x a month. Should really be 1x a week. Bad girl. You'll know that it needs washing when the foundation doesn't spread smoothly, or becomes streaky. Otherwise, it does a great job of helping the foundation blend into my face. It retails for $33.
The Armani brush is limited to just the Armani Foundation I have (Luminous Silk, review coming). The SA told me that the Luminous Silk Foundation (LSF) has some micro-fil tthing that necessitates the use of a brush, which I'm all for; plus I'd heard good things about the foundation, thus I thought I'd give it a try. Also, because of the foundation's ingredients, it's recommended that you wash it dishwashing liquid! Who knew? While unconventional, dishwashing liquid is readily available and it's cheap. The full size brush is $47, but I got mine in a set with the foundation, and it's travel sized.
I don't use the concealer brushes that often. If I'm going to a special event and want to get the red areas by my nose, I'll use a concealer brush, but it's not a staple in my arsenal. The BE brush is $20, Eco Tools comes in with set of 5 brushes for $14.99

Clockwise from left: BE Flawless Application Brush; Lancome Brush; BE Handy Buki Brush
For mineral foundations, these are the three brushes I use. The BE brushes came in the starter kit, along with the concealer brush mentioned above and the Bare Minerals foundations. I think the brushes are good for the money, although they seem to shed a couple hairs every time I wash them. Supposedly, the Handy Buki brush is for "smaller areas", but I find it works fine all over the face; maybe it takes a little longer for the 'swirl, tap, buff' to happen, but what's a couple extra seconds. The Flawless Application Brush is used primarily for BE's Mineral Veil, though I suspect it would be fine for other powders. It's hard to swirlp, tap, buff with this brush, but I find it's not as important with powder as foundation.
Mineral foundation is more popular than ever, so for all my drugstore purchases of mineral foundations, I use the Lancome Brush at the top. Honestly, I have no idea how much this brush costs, or what it's intended for, since it was part of a gift with purchase. It's closest to this brush on the Lancome website. $56? I'll take free. And yes, it does work with mineral foundations. Not to soft, not too hard, just right.
All right, enough of the goldilocks references.
Catch up on other posts about brushes:
Natural vs. Synethic
My Personal Collection