The Bell helmets at Walmart aren't especially heavy (about 80 grams more than a Giro Atmos though...) or uncomfortable, but the venting is *extremely* poor (which is to say it's like they don't have any vents at all). It's worth the extra money to not have your head drip sweat every time you take your helmet off.
I wore the Original Bell Biker in the late 1970s - talk about hot and heavy! Compared to that old turtle shell, my new Bell feels light as a feather and cool as a cucumber. As far as sweat goes, you're going to drip the stuff even if you ride bare-headed in the summer where I live! (Hint: Think 105F heat index and humidity high enough to swim in - in June.)