how to know the detail cost for RDS?

Hello.

There are two ways to view your AWS bill and one way to use AWS Cost Explorer.
If you want to check your invoice, please follow the steps in the document below to open the console and check the RDS items.
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/getting-viewing-bill.html

If you are using AWS Cost Explorer, please try operating as shown in the image below.
If you select "Usage type" for "Dimension" and "RDS" for "Service", you can check which items in RDS are generating charges.