More follow up.
Calculation you have going is based on assumption that any “incoming request” metric directly translates into “billable messaging operation”. That doesn’t seem to be the case.
I’ve looked into Azure Cost Management and Billing. Unfortunately, it doesn’t allow to isolate ASB cost of what you’ve already used. I assume you have more resources than just ASB namespaces to figure out the cost.
Saying that, there’s an Azure calculator to help you with estimates. Have you tried it? I’ve tried to see what an estimate would look like with the number of incoming requests as billable operations (event though it’s not correct to do as billable operations will be much lower) set to 1 million (roughly 2.5 times of what you’ve logged) and it was still $10 USD/m. After 14 million you start inuring some costs. And even then, for a million operations you’d be paying around 80c.
Hope this information helps.
Sean