For any business plan you are billed $3.00 per user per month based upon the number of users you entered in the subscription plan screen below.
Note that when you archive a user it frees up a slot for you to add another user. But archiving a user does not automatically reduce your user count. If you are removing a user and do not wish to add another user, archive the user first then go into the setup screen below and adjust your user count down.
Menu SetupSettings Application Billing tab Subscription Plans tab