A&C according to Flitestar 2hr flight at FL80 and FL120 gives 6 minutes difference. So are you saying that the extra 6 minutes of flight time are where the extra costs come from? My time to climb to FL80 is 11 minutes and to FL120 16.
At FL120 I am burning 28lph and at FL80 burning 33lph.
In the schem of things even allowing great fuel burn for the climb and the extra time for the flight 5lph fuel saving more than covers it?