Most jet transports have multiple engine choices. You can put dash 7s, dash 9s, dash 15s or dash 17s on 727s for example. What really affects the cost of the flight is the speed it is flown at. Long Range Cruise will normally give you the best return on the fuel burned. You can set a cost factor into your flight planning and balance higher fuel costs v. lower crew costs. Run the engine hotter and you'll pay more when it gets to the shop. Are you really going to be able to fly an extra section if you save 7 minutes on a two hour leg. Even if you cut an hour off a 14 hour leg, it's hard to make much use of it in a day. Over months and years it will pay off if you build a schedule around it. But if you do 6 two hour back and forth flights every day with a crew change in the middle, what do you gain? Hiring one more crew for a flight in the middle of the night?