I currently have three helmets:

- commuter helmet, usually cheaper Bell or Giro MTB helmet, $50 at most, don't care much about ventilation or weight
- a better helmet for mountain biking, lighter, more ventilation, $100 at most
- I just bought a very light, well ventilated helmet for road riding for about $100 too

I found that the cheaper helmets I used for commuting are kind of heavy and I start feeling their weight after a few hours of riding. Also the better helmets tend to have more comfortable straps and webbings that work better on longer rides. So $100 is a lot for a helmet but it does make a difference IMHO.