With Google Apps, you pay per account. So if you have 3 accounts you will pay for 3. Each account has it's own email address. However you can add an alias to an account. So
firstname.lastname@example.org will be the primary account email address and you can have
email@example.com as an alias where any emails sent to
firstname.lastname@example.org will go to the Gmail of
If you want to have separate Gmail accounts for
email2 you will have to pay for two accounts. Just note that each account come with their own Google Drive space and access to all the Google Apps Suite.
You can have multiple domains within the same Google Apps account in two different ways. Make sure you check out the limitations on multiple domains
1. Domain Alias
This will give each user in your current Google Apps an alternate email address with the secondary domain name -- No extra cost.
2. Additional Domain
This will be a new set of users added under the second domain name. Each user will have their own account and you pay per account.
More Info / Training - Google Apps Certified Administrator
I would highly recommend doing this training if you are going to be managing a Google Apps account, even a small one. The training material is free from Google and you can take the test ($100) at the end if you would like to have the certification on your resume or whatever. It explains the type of question you just asked very clearly and a everything else about Google Apps.