Billing is monthly, per workspace, and based on the number of active users you have. Active users are any users you have added which have not been deactivated.
Per-workspace billing provides more flexibility because it allows each workspace to be associated with a different payment method. For example, if you have 2 business units, you could create a workspace for each, and provide a different payment method (ex: credit card) for each as well.
To learn more about how billing charges are calculated, click here. To learn more about accepted payment methods, click here. To learn more about when to create multiple workspaces, click here.
Duplicate Users
If you create the same user in multiple workspaces, you will be billed multiple time for the same user (once in each workspace).