and in a way that they actually do have meaningful recurring costs per user to provide.

Yeah, it almost certainly costs them nowhere near $15/month per user. But, it's probably more than $1. They have "game masters" on call to help people who are experiencing a glitch or who are being harassed or something. They have ops people keeping the servers running, keeping the databases clean, rolling out the changes in a way that doesn't break things for players, fending off DDOS attacks and hack attempts, and so-on.

hosting isn’t magically cheaper because you’re in a country with a broken economy

There are probably some things that are cheaper. Like, they can probably employ Argentines to staff the Spanish-language game-master jobs cheaper than if they have to pay Americans. If they host servers nearby for low latency, it's likely that the datacenters and the staff to run them can be cheaper than the US equivalent. But, it's not going to be 15x cheaper.

OTOH, there are probably lots of things that are more expensive. I'd imagine that in a country with the problems that Argentina has, you're more likely to have brown-outs or black-outs from the power station. And the main headache will just be tracking the pricing for things. If some cosmetic add-on costs $2 US, you maybe have to revise that cost once every 3-4 years. But, if it costs 1000 Argentine pesos, you probably have to check it out every month or so. Plus, any contracts Blizzard/Microsoft has with any Argentinian partner, say for Internet service, probably has to be renegotiated constantly to deal with the latest inflation figures.