Mandrill has a simple pricing structure

Your first 12,000 emails per month are always free. After that, you pay on a per-thousand-email basis. You'll never pay for more than you use.

Example Volumes:

• 40,000 emails/month
• 100,000 emails/month
• 300,000 emails/month
• 700,000 emails/month
• 1,000,000 emails/month