If you are using CiviCRM’s built-in bulk email system, you may want to use a third-party SMTP system instead of sending large quantities of mail directly from your server.
There are a number of options, and Andrew Hunt wrote a blog post on civicrm.org about some popular choices. The post includes the general pricing plans and the costs at certain email thresholds, but we’ve now built a calculator for comparing your costs across all the options.
Note that some providers require you to pick a monthly plan (which includes a number of free messages), while others simply charge per message. Experiment with estimating above and below your typical number of emails to see what the range could be.