Every organization gets the greater of:
25,000 or 2,500 / user when using Ultimate,
20,000 or 2,000 / user when using Sales Professional, or
10,000 or 1,000 / user when using Sales Starter
If you need more than your free allotment of email credits per month, but only sporadically, pay-as-you-go pricing lets you send emails at your own pace without worrying about losing monthly credits.
(Purchased credits last 12 months. 1 credit = 1 email. Prices exclude regional taxes.)
The more you send, the less it costs. If you send periodic newsletters or other scheduled campaigns, monthly pricing gives you a better bang for your buck.
(Usage resets at the beginning of each billing cycle. 1 credit = 1 email. Prices exclude regional taxes.)