List price for a single MAX8 is $120M. I'd be surprised if $18M total of upside was the primary driver across an initial qty 40 aircraft order. They could have just offered that as an additional discount.
The 737-MAX800 is something like $10M more than a A321-NEO, when the A321-NEO meant retraining your pilots and the 737 didn't, that $10M difference closed a lot, now both are going to require retraining.