Is it safe to delete the restore DB to save Cost?

Hi Yes, it's safe delete when you had final snapshot. To be able to restore your deleted DB instance later with a final DB snapshot. The final snapshot is retained, along with any manual snapshots that were taken.

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_DeleteInstance.html#USER_DeleteInstance.Deleting

Let me know if this answers your questions. Thank you for your interest in re:Post community.

Regards, Manjunath