I don't have anything on my RDS, but it costs me every month.

Hello.

Is it possible that you are checking in a different region?
When you check your bill, you can check in which region the bill is being charged in the RDS field.
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/getting-viewing-bill.html

Also, a common reason why you are being charged for deleting an RDS instance is that the automatic snapshot created at the time of deletion remains.
So, from the snapshot screen for all regions, check all the tabs in the red frame in the image below to see if any snapshots remain.

Hello. I suggest to go to Cost Explorer. Then on the right panel filter for RDS service, dimension - Usage Type, granularity - Daily. Then you will see a better observation of what you are spending money on. Also you can select "region" in dimension, to see in what region you might have resources. It might be different region or snapshots, or proxy enabled, etc It can be

I have faced same issue in the past, please check replica/backup got created when you were deleting the database. Replica also cost you every month

Hello,

Sorry to hear about this!

You may be able to pin down those unexpected charges with this article. If not, please also reach out to our Billing team who's happy to assist further, within a support case.

— Ann D.