This is an interesting question. It can be very misleading, however, to look just at cent/mile. For example, an L fare (which is cheap!) from CMH-ORD roundtrip works out to about $0.25/mile. Given this I'd think that the short haul flights (and not one of the uber cheap ones like CMH-ORD) would have the highest cost per mile.

As for longhauls, I'm not sure.