This is a great question because I also held myself back from using Mailgun as it seemed so expensive.
As others have pointed out, you do not need to have a Mailgun subscription, even if their website wants to make you believe otherwise. That’s cuz they force you to subscribe to their Foundation trial at first. If you cancel your Foundation subscription, it will allow you to switch to the aforementioned Flex tier. That one is pay-as-you-go but free for the first 1k messages a month - the perfect option for a small blog and newsletter.