All helmet designs sold in the US are tested to the same set of conditions, so whether you buy a $30 or $300 helmet, you will get at least the minimum protection.

I recall a consumer magazine did their own tests on helmets a few years ago and determined that many of the lower priced helmets provided better protection than the higher priced helmets. This is probably because the cheaper helmets, with less ventillation holes, have more styrofoam.