An Azure relational database service.
Have you looked at Cost Management for your subscription in the Portal? That should tell you where the money went.
Do you have any database in the Serverless tier? Serverless is a two-edged sword. When used stringently, it can really help to keep costs down, but casual use can quickly become expensive. With Serverless, you only pay for storage when the database is paused, that is unloaded from any server. Great for development database that is only used occasionally. But if you have something that pings it every five minutes it will never auto-pause. And once it is loaded, Serverless is more expensive than provision General Purpose.
So for a predictable cost, it is better to stay away from Serverless. an S1 or S0 are great for predictable low costs, but you will have to accept that they not very powerful.