There are two separate bills, and people often only budget for one.
The platform
Klaviyo, Mailchimp, Omnisend, Brevo and the rest charge monthly, usually by the number of contacts or emails. Most have a free or very cheap tier for small lists. Prices climb as the list grows, which is one reason a clean list matters: you pay for every address, including the ones that never open.
The work
Someone has to plan the calendar, write the emails, build the templates and automations, check deliverability and read the numbers. That is either your time, a freelancer’s or an agency’s. It is almost always the bigger number.