The difference between inexpensive helmets and the more expensive ones is based on 2 things: weight and the number of vents. They both pass the same safety standards. The weight difference is small and the vent issue is not really that important either.
I agree that the weight difference is fairly small, but for some of us, the total volume of vent space is important. My head gets hot and sweaty easily, and I find that it's worth the money for me to get a helmet with more and larger vent holes. It makes a significant difference in how comfortable I feel once the temperature gets over 60 F. But it doesn't seem to matter for other people.