There is no objective scientifically-obtained evidence that more expensive helmets offer better protection than less expensive ones. (And trust me, I looked for it. If you know of any, I'd be happy to review it and change my mind if the methodology and reasoning is sound.)
A lot of people have crashed wearing an expensive helmet and claim that their injuries would have been more severe with a cheaper helmet. These claims cannot be taken as fact unless the individual repeats the crash circumstances exactly with a cheaper helmet and comes out of the ordeal with more serious injuries. This, to my knowledge, has never been done.
There is ample evidence that a full-face helmet offers much better protection than any other kind, but beyond that, there is no proof that more expensive automatically equals more protection.
Some riders ask if they should seek Snell certification in their new helmet. Although Snell demands higher impact ratings,
some question this approach. The argument being that the vast majority of crashes occur at lower speeds and engineering a helmet to withstand high-velocity impacts compromises their ability to dampen low-velocity impacts as effectively. But I have no opinion one way or the other on this and wouldn't steer anyone away from a Snell-certified helmet based on this alone.
I've love for an organization like Consumer Reports to do some proper cost/protection analysis to put the issue to bed once and for all. But until then, as far as anybody knows, the differences between low- and high-end helmets come down only to fit, design, and features.
Some other random points:
Don't forget that all helmets have a 5-year life span from the time they are manufactured. After 5 or so years, the protective liner (the stuff that actually protects your head) becomes brittle and is no longer as effective at protecting your head in a crash. At 5 years old, the helmet is "used up" and should be tossed in the bin, not stored for emergencies and certainly not sold on ebay.
Do not buy a used helmet. Ever. Not because you don't want to deal with someone else's old hair grease and sweat (although that would be a valid concern), but because you have no idea how old the helmet is and what the condition of the protective liner is.
Most brands have a good warranty. I would seek a lifetime (e.g. 5-year) warranty. At least one (AFX) has a crash replacement policy where they will replace your damaged helmet for free in the event of a crash.
This site is the best I've found for motorcycle helmet reviews. They're pretty meticulous and cover fit, finish, comfort, wind noise, etc.
