If they sold it at 1 dollar and half the people who downloaded it buy it, that's 20 million dollars before Apple's fees.
Super Mario Maker sold over 3.5 million worldwide on the WiiU at 60 bucks a copy.
It's also a mobile game with a few levels versus a fully fledged game made for a console, so profitability will be high. And that was just hypothetical. They could sell it for $5 and rake in probably just as many sales. Plus they haven't released on Android yet, and sales will keep tumbling in for years to come.. Super Mario Maker on Wii U has likely sold as many copies as it ever will.
Seriously, the hardcore Nintendo fans absolutely livid at the thought of Nintendo becoming a 3rd party, and the success they'd gain if they did so, seem more and more illogical by the day. I doubt we'll see it happen anytime soon, but Nintendo would make a lot more money, instantaneously, doing that now as opposed to bringing out yet another new console and hoping to god people buy it and make it a success this time round. And no, past success with the Wii does not absolve them of responsibility to make money now.