MailChimp has a straight forward pricing policy and that doesn’t include unsubscribed users. Irrespective of the plan you have opted for, MailChimp will only charge the users that are currently subscribed.
One misunderstanding that several people have is about inactive users. Inactive users are those users who have not opened the last 5-10 emails. MailChimp will still charge those users because they are still subscribed.
However, MailChimp has a clean list that it automatically creates for all hard bounces and 15 continuous soft bounces. This is similar to how a suppression list acts. This ensures that users who aren’t interested in your emails don’t receive it and end up costing you money.
So, in conclusion, MailChimp doesn’t charge any money for unsubscribed users.