I think with the sheer amount of factors, real world comparisons aren't going to come along very often.
You may hear a story of a helmet splitting or something, but the accident could have done it to any helmet.
SImpley because every accident is different
But to be honest, I don't hear stories saying, thank god I wasn't wearing a helmet.
To me, they have to comply with regulations (at least down here), so even you cheaper ones are going to be better than nothing.
I know one of the main reason people swear by the big brands (other than plain old snobbery), is the higher the price, a lot of the time the better then fit and feel.
Just one last point, I'm sure someone, a magazine, would have tested brands against each other in some sort of study in years gone by, surely someone has thought of comparing the cheap to the elite.