The challenge @SubstackInc will have, is that anyone who hits $25-50k in membership will quickly realize that giving 10% of your revenue (i.e. $2,500 to $5,000), is totally unnecessary... there are services like GHOST that charge $29 a month for a white-labeled solution

My guess is Substack is paying guarantees/advances to writers like @CaseyNewton (and/or waiving the 10% fee)

10% isn't huge at $1k a month, but when you hit $5k a month it's insane... $500 is a car payment

at $15k a month, 10% is a mortgage payment https://ghost.org/vs/substack 

I talk about the publishing world in an emergency pod last night

note: I'm not an investor in Substack, @patreon, GHOST (or anyone in this space).