According to this article, a self-published author named Mark Dawson earned $450,000 in one year from an email list of only 15,000 subscribers. And that was back in 2015 ... so, presumably, his email list is now much larger than that and so are his sales.
I think this man is probably an exceptionally shrewd marketer to be achieving results like this. But I also think marketing savvy is only half the equation. Writing quality is a huge part of it.
I think the self-publishers who quickly and haphazardly slap together an e-book and then post it to Amazon through CreateSpace won't see these kinds of results. On the other hand, if they write and publish several books professionally (e.g. including professional editing, graphic design, proofreading), then there is a good chance they can achieve these kinds of results.