All of the helmets mentioned in this thread do the job of protecting your head, so long as they have the appropriate SNELL rating, or whatever the newest/current rating is.
The difference between a $100 and a $750 helmet is:
1. Aerodynamics
2. Ventilation
3. Paint
4. Materials (Carbon fiber, Titanium, etc...)
In a crash both types of helmets are going to protect you in the same general manner. Where you notice the difference is Fatigue. A helmet with better aero, reduced wind noise, made of lighter materials will not wear you down on a long ride, this is where the extra money spent really pays off. A heavy, noisy helmet really makes for a miserable ride after any amount of time. And as far as paint goes, the more bad ass your paint job the more Horse Power it adds to your bike.
