Ghost would cost $288/month (or $241/month paid yearly).
Substack lets you get away with sending that many emails without attempting to monetize because you pay them in clout and with your reader data.
I think they are getting the better deal given how good your writing is and your reputation.
Ultimately, even if you own the email list, you’re sharecropping on their land.
They will email your list with their own promotional offers and marketing whenever they feel like it.
If we’ve proved anything in tech, it’s that consumers will fall for the trap of “free” every single time.