[ddeck]

>How much of the cost of a flight is fuel?

In terms of flight operating costs (FOC) per hour, about 16-36% according to the numbers for a 737-500 across for Continental/United/Southwest a few years ago.

It's very dependent on utilization. Fixed ownership (i.e. depreciation/leasing) costs start to dominate at low utilization rates.

Boeing 737‐500 FOC per block hour (~2017):

              Crew Fuel      Maint. Ownership Hrs/day
  Continental $510 $430(19%) $ 651  $698      3.9
  United      $927 $487(16%) $1048  $510      4.3
  Southwest   $388 $537(36%) $ 251  $350      8.2

https://www.icao.int/mid/documents/2017/aviation%20data%20an...