You see, that's where I'm a bit confused. The only data I can find is that cost/seat mile for the latter 737s is in the region of 6-8 cents/mile; Boeing claim the Max is a 20% improvement on earlier models. The 757 appears to be about 7.5 cents/mile.
https://www.planestats.com/bhsn_2014sep
Wiki hits it from a different angle saying that a 737 Max has a Miles Per US Gallon of 102/seat with the 757-300 at 88 MPG.
For sure, the 757 appears to be more expensive but not in the order of twice as much. It would be an interesting theoretical exercise to number crunch a 757 with LEAF engines.
