mac brushes are hand-crafted, with natural hairs. if taken proper care of, these can last years, even with regular use. i've had some of my mac brushes for 5 years, and they're still in perfect shape. i take really good care of my brushes, and they all look good as new.
i've gone through some cheaper brushes, and even some of the higher end, non-mac brushes, and you'd be surprised how noticeable the quality differences are. even the brush set, or SE brushes aren't as good. seriously, it's better to slowly build your full-size brush collection, as you can afford it, rather than trying to compensate with cheaper ones. my mac brushes are hands down the best.