Update: Mandrill has introduced this language in their Terms of Service:
If you use Mandrill to send bulk emails, in violation of our Acceptable Use Policy, then we may charge you at the comparable MailChimp pricing tier or terminate your account.
See Blog Post: Alternatives to Mandrill
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I've communicated with MailChimp and I'm told that CiviCRM users do not need to pay by the number of contacts they have.
MailChimp counts subscribers as contacts in their system that you would send bulk emails to. But CiviCRM sends each contact an individual email. MailChimp see this as transactional emails not bulk emails.
So (I'm told) that CiviCRM users can sign up for the lowest tier, $10/mth plan and send emails to as many contacts as they want. On top of the MailChimp fee, users will also need to pay the Mandrill fees, $20 per 25K emails.
So if someone sends under 25K emails per month they would pay $30/mth.
Update: I also noticed this: https://mandrill.zendesk.com/hc/en-us/articles/217467117
Am I required to import/move my Mandrill recipient data into a MailChimp list?
The number of Mandrill blocks purchased each month doesn’t need to match the number of subscribers in your MailChimp plan. The core functionality of Mandrill isn’t changing at this time, and the recipients of your transactional emails sent through Mandrill are not required to be subscribers in your MailChimp account. Any paid monthly MailChimp plan will satisfy the new requirements.