Very very little. It’s a billion tiny little bits of text, and if you have image caching enabled, then all those thumbnails.
My personal instance doesn’t cache images since I’m the only one using it (which means a cached image does nobody any good), and i use somewhere less than 20gb a month, though I don’t have entirely specific numbers, just before-lemmy and after-lemmy aggregates.
As with all things email, they probably really wanted to make sure that the mails were delivered and thus were using a commercial MTA to ensure that.
I’d wager, even at 20 or 30 or 40k a year, that’s way less than it’d cost to host infra and have at least two if not three engineers available 24/7 to maintain critical infra.
Looking at my mail, over the years I’ve gotten a couple hundred email from them around certificates and expirations (and other things), and if you assume there’s a couple million sites using these certs, I could easily see how you’d end up in a situation where this could scale in cost very very slowly, until it’s suddenly a major drain.