https://www.nytimes.com/2021/04/11/business/media/substack-n...
This writer had 18k subs at $50/yr ($90k/yr revenue, of which Substack would take 10%) but was paid $430k for 2 years. Maybe they grow their reader base to close some of that gap but it's still a loss leader to build reputation. It's possible some other writers took less favorable deals but I haven't seen the details.