Programmer and newsletter author Molly White has a complete write-up here comparing newsletter engines Substack, Ghost, Beehiiv, Buttondown and Mailchip. She’s shared exhaustive spreadsheets comparing the costs at different subscription levels.
In short, “Cheapest option is self-hosting, though of course there is added time cost and a technical barrier to entry. I pay $100–$150/mo to self-host Ghost with a bit over 25,000 subscribers.For free newsletters, Beehiiv is free for up to 2,500 subscribers. ConvertKit is free up to 10,000.”