One of the major reasons that the expensive helmets (some Giros, especially) cost so much is advertising! They spend many $$ sponsoring teams in order to have their name seen.
Several years ago, Consumer Reports tested the protection of various bike helmets (not the style or durability, though). The highest rated helmets were the cheapest, taped-on-cap style Bell helmets, costing about $30 each. Many of the high-zoot $100+ helmets were rated not acceptable, due to chinstrap latches that failed in testing.