How much would it cost them just to keep their own store running? Could not be much on a virtual machine with no content changes. And they won't need exorbitant capacities. There will be no download waves just because a new game gets on the market, after all.
Based on some similar experience: low millions per year.
Skeleton crew of devs+ITS to keep the lights on, software licensing, hardware costs, customer support, financial oversight, legal oversight and occasional compliance efforts.
Probably a good amount of technical and organizational friction involved, too, for whatever services, knowledge bases, and corporate policies were shared between the Wii U and Switch.
It seems stupid when you compare it to the fact that Pretendo probably pays about 1% as much, but that’s just how businesses work.
I understand they pulled the plug, what pisses me off is them being assholes and saying that you can't look for an alternative to a service that they decided to kill.