I don't want to discourage anyone from buying the most expensive helmet they can afford, but still I'd like to mention the article posted here. It's an interesting point of view and a lengthy article to be sure, but something you might want to consider.
So what do I wear? It's a Bell Sprint I bought for about $150, the reason being that it's a good fit and a good helmet. And there are others I'm sure. Maybe someday I'll find an expensive helmet that's a better fit, has better ventilation, and is quieter, which the more expensive helmets usually are, and I'll get one. For now however I'm confident with what I have.
Just a thought, for what it's worth. You should buy a helmet that you feel good about.