This article reports that airlines average 49 revenue-passenger-miles per gallon of jet fuel. That's about 20 gallons per 1K miles/pax. Jet juel is about $2.10/gallon
recently, so $42/pax/1K miles.
If the average pax + bags weighs 200 lbs, that's 21 cents per pound/1K miles.
Not sure how cargo fits into that equation.
And of course, that's not the marginal cost - the empty plane itself is heavy and that cost is buried here. But it's a ballpark starting point.