[skyking]

The cost is not so important for tracking usage. Don't even put it in there if you don't want to. Do you have a canopy? You could see some benefit with a canopy and a deflector at the back of it to move the air up onto the TT.
More ambitious is a folding boat tail for your trailer. It would net you the largest gain.
Look in the aerodynamics forum to see what the commercial truckers are doing. They have these simple little boat tails that fold quickly. They would not do it if it did not pencil out for them.