Those of us who drive a lot already pay. Only the first tank is free. And some manufacturers don't even fill the tanks before lending it out again. Borrow a big-engined sportscar and you'll be paying through the nose for a week's worth of driving, unless you hypermile... which is frustrating on something with three hundred to four hundred ponies.
Frustrating because hardcore hypermiling nets you the same economy you'd get in a Mirage if you were pinging off the rev limiter everywhere.
