From what i am reading only mario party jamboree and kirby upgrades will cost money, the zelda ones are free if you pay for switch online and metroid and pokemon dont appear to have any proce attached to their upgrades.
The running theory is thst most games wont cost to upgrade unless they offer expanded content. But expansions costing money is nothing new.
That's $4.17 a month. It's even cheaper if you get a family account and share with your family and / or friends like i do. (I am actually UK, but it seems the US price is the most quoted.) And comes with all the n64 (and the new gamecube) games. Its the cheapest online service of the three main consoles.
Also, to be clear. Im not a big switch guy, and i think nintendo are a greedy company. I mostly play playstation these days but my switch is handy and there are some amazing first party games.