Flights, I agree.

Passengers? Maybe not (though an economist might disagree).

Most of the costs for an airline are fixed costs, meaning that a given flight costs a certain amount regardless of the number of passengers. Incremental cost per passenger is quite low, especially with online booking & check-in. You've still gotta have the ticket & gate agent, the cargo loader, the pilot, etc. So... the incremental cost of a passenger on a flight that's not full is very small compared to even a low fare. Assuming you've got enough passengers paying enough money to cover the fixed cost, the rest is gravy.

That's one perspective.

The other is to allocate all the fixed costs across the passengers on the plane. The cost per passenger varies quite substantially under that model, meaning a plane with 1 passenger at full fare would still be a "money losing passenger". That's the model that most airline folks use when talking to the public. If the plane's full, you can say that the incremental low-fare passenger is a money-loser.... but the fact is that the same passenger actually adds to the profit as long as his/her incremental cost (fuel, ticketing fee, etc). A low fare passenger is money-losing only when said passenger takes the place of a higher paying passenger. And even then, it's an opportunity cost, not outright money loss from carrying the low-fare passenger.

Were the airline to outright decide not to operate an lightly loaded plane unless a certain number of passengers board, then the fixed cost becomes variable and the discussion changes.

what's the famous statement? Lies, Damn Lies, and Accounting...