Consumer Reports tested helmets in 2007 and found some differences in impact absorption (Bell Citi, Bell Slant, and Specialized Aurora rated "very good" vs all the others which rated "good"). Of course, they only tested a handful of models, so it's not like those are the best 3 helmets in the world. Also, I wasn't super clear on what the amount of difference was--maybe they're all almost the same and they decided to count a small deviation as making a difference.

But those two Bells can be had for less than $50, so you definitely don't have to spend more than that.