You're right, it's adding a lot to the total stadium cost, but the long-term aim is to permanently host an NFL franchise.
That may or may not happen, but Levy is gambling that it will, which is why everything (not just an NFL pitch, but an entire suite of separate NFL facilities) is being put in place to make it possible ... and to make it possible without impacting negatively on Spurs as a football club in any way.
If does happen it will be hugely lucrative for Spurs. But even if doesn't, even if it's just a few NFL games each year, there are still also the various entertainment events - rock concerts etc - that will be enabled.