Thanks for posting your question in the Microsoft Q&A forum.
Azure Service Bus does not charge for storage, but there is a base charge for each Azure subscription that has a provisioned Service Bus Standard namespace. This charge is not associated with specific Service Bus resources like queues or topics but applies to the overall provisioning of the Service Bus Standard namespace in your subscription.
If you have a Service Bus Standard namespace, you may see line items such as /subscriptions/<azure_subscription_id>/resourceGroups/$system/providers/Microsoft.ServiceBus/namespaces/$system
under $system
. These charges represent the base fee for having a Service Bus Standard namespace provisioned in your Azure subscription.
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful