Brian, with today's gas prices and the average mileage the Gen I's get, I figure a dollar every 10 miles I ride. I also ride an 1150RT and a Dyna 96ci Custom. All three get get around 40-42 MPG, so the price per miles on all my bikes are about the same.
The C14 is my favorite ride, and if I could get 50 MPG on it by some mods, it may be worth the modding you are considering. Had a KLR650, and it got 50+MPG consistently, but 70+ MPH was pushing it. That level of mileage on a 14 would be FINE....