err sorry umm rather "what are the financial implications of a 250 seat aircraft against a 219 seat aircraft"
The financial implications (just on the fuel burn) are that with an extra 31 fare-generating seats the fuel penalty is not signifcant (within certain parameters).
And this is where your comparison falls to pieces half way through.
You compared two different layouts which command different revenues in different markets which is an assumption with an error factor well beyond the difference. It also assumed all other costs are the same which is for sure wrong and every seat is sellable without having to cut sales revenue on each seat.
The comparison by anm at least allows the airlines to then work out how to divvy the costs. Doing it by seat shows nothing as the value of each seat is not similar.