While pricing varies by provider, the cost structure is usually the same. Most email marketing providers charge a monthly fee based on contact list size, with a range of packages and plans. With this structure, most providers allow you to send out an unlimited number of emails each month and have enough different plans to fit into the budget of any business.
Mailchimp offers a forever free plan which allows you to send 12,000 emails for up to 2,000 subscribers. This plan is fairly limited because you don’t get features like send-time optimization, advanced segmentation, multi-variate testing, etc. You are also required to display their branding in your email. Last but not least, support is restricted to email only which is fine if you’re a hobbyist but as a serious business, you need reliable support that you can count on.