Hi Ryan Brown
Typically, there isn't an additional charge just for using the Billing API itself. It's considered part of the management services that Azure provides as part of the Azure Management APIs.
The Azure Billing API may impose rate limits on the number of API calls that can be made within a specific time period.
There may be rate limits on how many requests you can make in a given period per user. Please refer ot below link
Please let me know if you face any challenge here, I can help you to resolve this issue further
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