"Gary Drescher" wrote in message
news:BA_uc.36617$Ly.26936@attbi_s01...

Why is the marginal cost what's relevant (rather than a pro rata
share of the total cost)?


Pro rata works fine if all users are equal. What's a Skylane's share of a
runway built to support 747s?



If I fly on an airliner, or I step onto a bus or subway, the marginal cost
of my presence (in terms of the extra energy expenditure)
is a negligible fraction of the fare. Should I therefore expect to be
transported nearly for free?


Of course not. You get one seat, the same as every other passenger.