Most helmets are made of styrofoam, an insulating material, so heat must be carried away by air flow over the head. Consumer Reports rates coolness for the few helmets they test. A lab study showed that coolness depends mostly on the size of the front vents, but it is affected by head shape, length of hair and other variables. Oversize vents cut down on the area of foam in contact with your head, concentrating the force of a blow on a smaller area of your head. We don't recommend them, since most riders don't need huge vents and do need optimal protection.
Prices are low now.
They all have the same CPSC certification since 1999, but cheaper helmets tend to be plainer or lack a rear stabilizer. There are mail order suppliers who discount the popular brands. Our Web page and Toolkit list other sources, mostly for helmet promotion campaigns. But we recommend buying your first helmet at a bike shop, for help with fitting. For best protection you want the helmet level and low on your head.