About $5-10 will get you a good quality brush that will last you quite a while.
No need to spend ridiculous money on brushes.
I know some people who have spent over 50 bucks on a size 2, and has no advantages over a $5-10 brush.
Whatever you do though, do NOT buy cheap brushes.
I bought a set of 5 brushes a while ago (size 000, 0, 1, 2, and 4) for about $8, and ended up throwing them in the bin.
Stick to around the $5-10 mark, take care of your brushes, keep them clean, treat the bristles with care, and they will last you quite a while.