If no one flies, the airline makes nothing. The extra gas cost of a passenger is almost nil. Ticketing & food are the 2 major incremental costs of another passenger.

On one of my $59 LAX-Buenos Aires flights, the plane was so empty that I had two whole rows in coach (in the center) and (near the window). They probably should have sold more $59 tickets!

...and yes, in the long run, an airline needs 5-10 cents/passenger mile to break-even. ($600-$1,200 for this flight)

The very profitable European EasyJet and RyanAir have shown how they can use revenue management to give away seats for $0.01 (or even less than 0, -1 GBP is the best sale I saw) and yet maintain 20-40% profitability in some quarters.