In order to be sure your head will be protected it is important to check for either ANSI or SNELL certification. All helmets made by Trek, Specialized, and Giro are rated for certain. I think all Louis Garneau and Bell helmets are rated, too. Other brands are also, but I can speak for them with any certainty.
Decent helmets start at about $50-$60. Helmets that have a layer of carbon fiber between the foam core and plastic shell can have larger vents and still offer good protection.
One important factor is comfor of fit. Some helmets are better for people with roundish heads while others are better for narrow heads. Never buy a helmet without trying it on and fitting it for your size, first.
Ventilation is great in the summer. In the winter you can wear a headsock under the helmet or a cycling cap.
I like the Specialized S-Works Road. It's not inexpensive (about $200), but by far the most comfortable lid I have ever owned.
Good luck, and I hope you will never need to test the rating of the helmet yourself!