IIUC, they valued it at 1M per plane [1] and I guess list price of the Max is ~100M [2] so while 1% of the aircrafts value isn't nothing I think it gets completely dwarfed by operational costs.
Not sure how expensive their fuel but I have to imagine over the course of the plane's lifespan that fuel is going to be the primary cost (maybe maintenance / parts would be higher). So if you have to retrain your pilots to fly a plane that requires less fuel per passenger I think you'd come out ahead.
I think one of the biggest drivers here is the opportunity cost of taking pilots out of flying schedules to provide training on a new platform, and I imagine this is exacerbated by the ongoing pilot shortage.
Also there are operational efficiencies captured on the maintenance side of things, more streamlined parts inventory management, etc.
Then you also have to rework all your routes and contingencies. If you now fly more than one type of plane you have to make sure you always have a pilot available for right type of plain in a specific location if you ever need to restaff a route.
That’s all to say that adding a new plane platform to an airline is probably akin to a full replatforming of a production app. Sounds simple but there are tons of nuances with the actual execution.
It can create a lot of logistical problems if your pilots are qualified for different planes. Say your 737 Max pilot gets ill and you've got some 737 normal pilots sitting around but no Max ones. If they are not allowed to fly it and you have to cancel that costs big.
The CEO of Southwest wanted that. They buy only one kind of airplane, B-737 variants. This simplifies repair, parts, training, and staffing. Hence the 737-MAX, and the problems of the 737-MAX.
The name Airbus is disturbingly apt.