Honestly? I said it before in another context. There was a year when I overlapped a $40 helmet and a $180 helmet. There was about a 20% difference between them. Even if it's a matter of literally overheating, I don't know that that's the best use of your money.

On the MC, an evaporative cooling vest has saved my bacon (from turning into bacon) more than once. I don't know how well it would do at the lower speeds of bicycling, but I know mountain bikers use them. Or if it works for you, you could just wet down your hair, or wear one of those evaporative bandannas under your helmet.