What are the variables that go into the expense of a plane flight? What are ways of minimizing that cost?
Distance, plane Weight, and plane Speed are the 3 variables used to determine a plane's flight expense. (The higher these values, the higher the total cost.)
You can spend "Bux" to upgrade an individual plane to lower its Weight 3 times, by 5%, 10%, and 15%. These upgrades names (in game) are slightly misleading: they represent your total weight reduction, so each upgrade is only giving 5% for a total of 15% weight reduction, and 15% reduction in cost. See "Napkin Math" below for details...
Please note that increasing a plane's Speed via upgrades will increase flight costs, but lower flight durations.
Also note that adding/removing passengers and cargo currently has no impact on a plane's weight. (as of v1.0.3 anyway)
I tried out a few Speed and Weight upgrades... Each time I lowered my plane Weight by 5%, it consistently lowered the route cost (from original cost) by another 5%. Increasing the Speed by 5% increased the route cost by 5%. When I combined the two, the results showed the effects multiplied.
Thus, the game appears to follow a simple formula for cost:
Weight(W) * Speed(S) * Distance(D) * SomeConstant(SC) = Cost(C)
So, if we reduce weight by 15% and increase speed by 5%... (0.85*W) * (1.05*S) * D * SC = 0.85*1.05 * C = approx. 89.25% change in Cost.
Weight is a variable that does make a flight cost more. You can minimize it by buying the bux upgrade to lose weight for each plane.