Each month AWS charges your payer Root Account for all the linked accounts in a consolidated bill. The following illustration shows an example of a consolidated bill.
Reference Architecture AWS Organizations features
AWS Organizations Billing FAQs
What does AWS Organizations cost?
AWS Organizations is offered at no additional charge.
Who pays for usage incurred by users under an AWS member account in my organization?
The owner of the master account is responsible for paying for all usage, data, and resources used by the accounts in the organization.
Will my bill reflect the organizational unit structure that I created in my organization?
No. For now, your bill will not reflect the structure that you have defined in your organization. You can use cost allocation tags in individual AWS accounts to categorize and track your AWS costs, and this allocation will be visible in the consolidated bill for your organization.
Source | AWS Organizations FAQs