I remember seeing it's like 4%, but I think Nintendo should be above that.
On average it never hits 2%.
Consider this, that 2% generally spends more than $10 on the game and a portion of that 2% subsidizes the other 98%.
Nintendo do doesn't want that whale mentality for MR, which is respectable.
Now I think Nintendo will have IAP games, but they will have it for games where IAP doesn't damage the value of actually playing the game.
I do not think you could do a Mario Platformer with IAP and not damage the value of the gameplay.