There's a surprisingly high price floor on airline tickets. Like $30 (one way) in fees to the government and the departure airport. Plus the incremental fuel [0] and any complimentary consumables, etc.

Not much of a factor on more expensive fares, but for short flights where a $50-100 fare is realistic.

[0]: even with today's oil prices, the extra fuel is only $20-30 for a transcontinental US flight. negligible for short flights.