All the advice thus far sounds good; sorry if I am duplicating any.
As far as I know, Arai and Shoei are the leading brands. The differences within the manufacturers themselves, i.e., an Arai helmet w/ fancy graphics vs. a solid one is self-explanatory. Generally speaking, the reason why some of the helmets cost more is because of the fancier graphics and for the comfort of the helmet, as someone aluded to earlier; better ventilation, lighter composition, etc. Among the price difference between the manufactures, i.e., Arai vs. KBC, it's the brand that you're paying for; however you want to interpret that. For anyone who just says you're paying for the brand in the difference, they're definately right. My point would be that there is a "real" good reason why the higher end helmets, i.e., Arai and Shoei commands that kind of price. I don't believe in free lunch. But, keep in mind, that as long as the helmet at least have the DOT, and SNELL label, you're head should be in good hands. As far as using websites to save money; that sounds good. The trick is, to visit the bike shops and try out the helmet there. It's similar to purchasing apparel on-line vs. going into a shop. I, normally wouldn't buy on-line w/o first knowing how it was going to fit. Common sense. Once, you're satisfied w/ THE helmet, call around and locate the bargain on either through the websites or ads listed in the bike mags for less.