This is related to the tenant cost. The answer is the same – nothing specifically. You don’t pay for Office 365. You pay for subscriptions within the Office 365 family. Power BI Pro is an example of this. Office 365 E3 or E5 is another example of a subscription that you would pay for.
You pay for subscriptions and then assign licenses from that subscription to users within your tenant.
Power BI (free) is not a subscription. It is a free license block.
So, if you have Power BI (free), add some accounts to your tenant and assign licenses to those users, you aren’t paying for anything. You can get to the Office 365 Admin center to manage those items. You don’t pay for use of the admin center.
You don’t pay for Office 365. You pay for subscriptions!