I was planning on buying a vented tailgate this year, but will not now. What changed my mind was a horrific episode that occurred to me while driving down from Stroudsburg after camping at a friends house.
Since the drive up there takes a little more than half my available 28 gallons, I planned to top off with the two 10 gallon plastic diesel cans before I drove home (avoiding a refuel stop on the way). I store my kinda squarish shaped 5 gallon cans up against the tail gate. I normally tie them down with a tie down strap to deter "casual theft" but after I gassed up I forgot to tie them back down. I noticed when doing my walk around, but I figured they were not going anywhere since the camper was right there about 6 inches above them.
I was about 55 MPH on the PA Turnpike when I looked back and saw the two cans "flying" under the camper's overhang. They were all over the place and in danger of flying out at any moment. I had to pull over and tie them back down.
Apparently, the aerodynamics of the flow around the front of the camper results in "dead air" in the bed of the truck with a solid gate. The faster moving air in the space above the bed causes a lower pressure above the cans in the bed that lifted them up and nearly out.
I thought about that a lot. When I have my soft cover over the bed, it is quite obvious (from the vibration of the top) that air coming over the top of the truck is hitting the last 12-18 inches of the cover right before the tailgate. This was the reason an air gate was in my future plan before the Turnpike episode.
The dead air in the bed of the truck results in a "smooth" airflow and less drag than all the junk I typically have in there. With an air gate there would be no dead air and higher drag. I think if I had a bare bed, the airgate would have lower drag (some) and "might" save enough gas to justify its high cost over time.
The payback is "iffy" at best in my situation and not worth the cost to find out.