Hard to say because I write very sporadically on Medium anymore. I’ve been dedicating a lot of time that I could be spending writing to a new business I’m starting-up and my two kids.
New content generates more $. I can make around $700 on months when I write a good bit. When I don’t write much, the existing stuff brings in around $300. I think it would be pretty easy to make over $1k per month if you wrote at least four or five good articles weekly consistently.
If you treat Medium like a job and write 4+ hours daily and know how to write well, I believe it’d be possible to earn a reasonable living here after you get established. I would not recommend that though because they could pull the rug out from under this whole model at any time.