The Tegra X2 is not much more expensive than the X1 I don't think. The Jetson X2 is the same cost the Jetson X1 was for the last two years.
It's simply technology scaling down in cost, and that's not magically exclusive to the chips Apple is using either.
Nintendo more likely has a certain period of time they need to use the X1 due to a contractual agreement ... possibily, but I don't think it has anything to do with cost.
I would bet money right this very second there are Nintendo Switch units in Nintendo's R&D lab that have the Tegra X2 processor inside of them, already running.
Nintendo is actually probably already talking to Nvidia about the Tegra Xavier chip for the Switch successor, they likely already know the X2 inside out and it's just a matter of time before they switch over to that. Xavier is actually probably what Nintendo's R&D is right now looking most at and spending their days focused on.
I'm not sure the X1 is even that cost efficient to be honest, it's using a 20nm manufacturing node, which is a very niche manufacturing process. The 16nm that the Tegra X2 uses is much, much, much more mainstream, so eventually it may become actually cheaper for Nintendo to use an X2 instead of an X1.
