Mandrill is a paid add-on for any Mailchimp Monthly Plan. Users will purchase "blocks" of 25,000 emails at the beginning of their Mailchimp plan's monthly billing cycle. The price of each block starts at $20, but decreases as you purchase additional monthly blocks. You can use Mandrill's Pricing Calculator to see how different volumes affect the monthly price of Mandrill. These blocks expire at the end of each billing cycle, so it's important to estimate your transactional email volume as closely as possible.
You can configure your account's behavior to help prevent any disruptions in sending if you exceed the volume of purchased email credits. After setting a threshold of remaining email credits, you can receive an alert or set up your account to automatically purchase additional top-off blocks when the threshold is reached.
Any Dedicated IPs configured for your Mandrill account will incur a recurring $29.95 monthly fee. For more information about Dedicated IPs, see About Dedicated IP Addresses.
You can view your Mandrill billing information and history in your Mailchimp account.
Mandrill counts each recipient as one email for credits and billing. For example, if you send one SMTP message to five recipients or include five recipients in your API request, that counts as five email credits toward your monthly limit.