Go ahead and spend a small fortune. It's your head and you only have one.

I always buy Shoei helmets and I always buy the best I can afford. Right now I have a Shoei Syncrotec ... also has flip-up chinbar. Very handy. Whatever you buy make sure it meets or exceeds DOT safety standards and is SNELL approved.

I know HJC has some lower-cost helmets that are very good quality. Try here:
http://www.chaparral-racing.com/
but make sure you are properly fitted and know your size. If your helmet doesn't fit you might as well not wear one.