More expensive helmets have more advanced retention features like Specialized's Brain Trust 2 system. They have more expensive manufacturing processes to mold the hardshell and styro together or more as one unit. Cheap helmets usually have the hardshell "skin" stuck on top of the styro.

They have more sophisticated and better looking graphics, materials, finish and are usually the best a particular company can make.

Proper fit and comfort are most important as any foam helmet will protect about the same. However, an ill-fitting helmet will not do it's job so if you can only get a good fit with a $100 helmet, buy it. If you get a comfortable helmet you like that fits well for $30, than buy that.