When using Amazon Relational Database Service (Amazon RDS), you can now view detailed billing at the database instance level. This information is available for RDS automated database backups and manual database snapshots, and can be viewed in AWS Cost Explorer and Cost and Usage Report (CUR).

from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/10/amazon-rds-enables-detailed-backup-storage-billing/