With NationBuilder's blast email service you pay for the number of email opt-ins in your database. The the price of your blast email service is a monthly charge that is additional to whichever plan you are using.

For example, if you have 75,000 people in your database, but you only have emails for 5,000 of those people, you are paying $49/mo in addition to your basic plan.

How many times you email people during the course of any given month will not affect the price of your plan, only the number of people with emails in your database will.