What do people charge for monthly subscriptions?
That’s completely up to you. Lots of people charge between 5 and 10 dollars for a weekly podcast, some charge less. Some charge a lot more money per month if the content is extremely valuable, or the audience is well-funded. Consider that radio traditionally charges around 10 a month for private “listener clubs” where they have access to daily content, while other podcasts on business strategies and high level marketing strategies might be worth $100-300 a month. You get to set the price, and see how your audience reacts.