I bought a $20 Bell from Walmart yesterday. I was in a motorcycle accident at 55mph a number of months ago, helmet saved my freaking life. While I wholeheartedly believe these things are goofy looking, you won't see me ride anywhere significant(read: outside the neighborhood) without one. But I'll never be able to justify spending more than 50 bucks on a helmet.
I'll replace it when it gets raggedy, nasty, or broke.