I've put vents in the hood of my 01 Blazer, and although their intended purpose was cooling safety margin when I'm in 4L in Arizona heat, they also happened to dramatically increase air conditioner effectiveness and have netted me a scangauge-verified 0.5-0.9mpg increase (across 3 tanks of fuel) - which I think is largely attributed to MUCH less frequent engagement of the fan clutch.
Cost:
Hood Vent Inserts from 80's turbo chrysler new yorker: $7.50 at Pick N Pull
Dremel Metal-Cutting Rotary Bit $10 (already owned b&d rotary tool)
Screws/washers/nylock nuts $3.75 at Ace
Placement was determined by 8" strips of yarn taped to hood, and by visualizing air flow under the hood (basically making sure with my eyes that there was a clear/easy path from rear of radiator to the vents, as a vent blocked by some part of the motor wouldn't do much good) - they ended up needing to be about a foot ahead of the windshield, and out to the sides. Farther forward the hood was too curved for the stiff magnesium vents, farther back the yarn snaked around on the hood at high speeds, telling me there's some pressure present. Closer to the middle there's engine parts right under the hood. (I guess they could have gone a bit closer to the middle...)