I think there's a thread on here on "Larry the Garage Door Guy"
and he charges around $100 for vents. I opted to have the Solar Guys
install a GF-14 garage fan, which includes 2 garage door vents.
It pulls a breeze that cools down the garage and also vents/cools the attic.
I'm really pleased with it, ran about $525 as I recall which was a great price
from what I researched online.
Even vents have to help a ton though, as does leaving the attic "hatch" vented.
Little Rock, AR
Village of St.James, The Villages, FL