It's always seemed to me that bici helmets are like champagne - you DO get what you pay for, but price increases logarithmically against features/quality.

I'm very prone to overheating, but I'm also fine in a $60 helmet (Spec' fits me well). I do a lot of things to make sure I don't overheat - see the thread on heat and humidity - but when I had a $200 helmet, it just really didn't make that much difference to me in that department.

YMMV.