I read an interesting report by a Professor on the internet back last year saying that cheaper helmets might very well be safer than the more expensive ones. Apparently its all to do with the Polystyrene used as protection on the inside. The cheaper Polystyrene gives more than the more expensive stuff thus in an impact reducing the trauma to the brain in a knock.
It made for very interesting reading and said that sometimes the more expensive ones can't justify their prices when in the tests they did on the helmets they came out the worse. Albeit theres other considerations to consider like fit, wind noise, quality of the kit, customer satisfaction, backup etc. I guess its all down to personal preference really isn't it.