It's worth spending money on your primary brushes.
Ages ago someone gave me some really good advice: spend the most you can on the paintbrush that you use the most. It really is worth the extra money.
A good paintbrush - the best one you can afford - will last longer than a cheap brush. It will have better spring, more pliancy, it will not shed bristles, and it will balance perfectly in your hand. Anyone who has ever painted anything has, at one time, struggled with a cheap paintbrush. They are forever shedding bristles in the middle of your work, and are either stiff when you want a flexible fiber, or too floppy when you want a stiff brush.
I admired this rule anew recently, when I was moved to go excavating through my supplies to uncover my old watercolor things. Many of the cheaper brushes I had were basically unusable junk by now. Some had shed so many bristles that they were rendered useless. Others had rusted through their "brass" bristle-holder. But not my most expensive brush. It was ready to go, just as good as the day I bought it.
My favorite brush for watercolors is a #6 with an oval tip. After going through several cheaper brushes of this size and shape, I finally shelled out the cash for a Winsor & Newton Sceptre Gold, which is a sable/synthetic blend. Hardly the most expensive brush out there, but it was a lot of money for me at the time.
This brush, far more than any of my cheaper brushes, is a joy to paint with. And it has outlasted dozens of its cheaper peers. Although it seemed like a near-ludicrous expense at the time, it has proven to be a bargain.
The only exception to this "buy the best brush you can" is for brushes that you use rarely. I (and I'm sure many painters) have many brushes that I bought because I needed them for one specific effect, and I hardly ever used them past that one painting. It's not worth your money to shell out for expensive brushes you won't use more than once a year.
I also recommend that beginning painters go through a lot of cheap brushes before buying a nice one. There is no one right brush for everyone, and you don't necessarily know up front which brush size and shape will turn out to be your favorite. Once you know what kind of brush you use the most, go shopping for a good one!