About a year ago, I stumbled on Substack's page that shows writers what they'd make, net of fees based on number of subscribers * price. 400 subscribers at $10/month works out to ~37,000/year. That's not a way to get rich, but it is almost exactly the US median income, and not far off the media journalist salary.[0] If you're willing to write well, reliably, the math of paid newsletters seems pretty friendly for making an ok living.

[0] https://www.payscale.com/research/US/Job=Journalist/Salary