The last thing I would worry about is whether it would pay for itself. I want something that will keep the air out of that space and reduce the noise.
A year ago I was heading west on I-80 across Wyoming. There was a storm system approaching from the west - the freeway signs warned of steady winds from the west at 50 mph gusting to 70. The speed of the truck combined with the speed of the wind - well over 100 mph. The noise from the air caught between the cab roof and the camper was horrendous - the air pressure was actually making the cab roof vibrate - flex up and down. If an air dam could prevent that I'd sure give it a try.
