Well, say the A321LR can do 90% of what the proposed 797 can do. Being as it is already developed it will be cheaper than the 797. It will also be cheaper to operate, as it is smaller, single aisle and up to date with latest engine technology etc.
The Airbus A321LR can do right now 90% of the flights performed by the Boeing 757 (at a lower cost in comparison with the 757), but that does NOT guarantee a priori
that the A321LR will do 90% of what the proposed 797 can (the 797 will also touch the 767-200 segment).
For the 10% of Routes I give you a A330 for dirt cheap for every 10 A321LR you order. Is it too big? Yes. But the price will be lower than that of the 797. Again, because it is already developed and because I make my money on the A321LR sales. Is it more expensive to operate? Yes, but due to the lower aqcuisition cost you can operate it for many many years until you reach a break even point.
said, the A330 (and the A330neo) is the same size as the 787 Dreamliner.
So, the A330neo (or the A330ceo) is too much plane to be considered as a good replacement for the 757 and 767 (at least the 767-200). And as what've seen, the A330neo, although is a good airplane for the airlines which want an airplane to replace their A330s at a lower cost (in comparison with a 787/A350XWB), currently is in a delicated situation in question of gaining new orders in comparison with Boeing's 787 (which is helped by Boeing's decision to ramp up the production of the airplane).
Even if the MOM Market is 1000 planes. Spreading 20B$ across 1000 planes is already 20M a piece that you can't discuss away..... Is it half that, then 40M etc.
That assumption ignores that the 797/NMA will use many technologies developed originally for either the 777X or the 787. So, they can use the same technologies to develop a clean sheet 757/767 replacement with capacity up to 250 passengers and a range up to 9200km for LESS than $20 billion which you're claiming.