Would like help in determining the cost for deleting all objects in a bucket

There is no cost to delete objects in S3, but the method you use to delete 21M objects may incur cost. Assuming we want to delete all objects within this specific bucket, and that versioning is not enabled on this bucket, the simplest method would likely be to use an S3 Lifecycle Rule to purge all of the objects.

When using S3 Lifecycle Rules, we are not charged for API operations in S3 (such as a LIST operation), but we are charged for the lifecycle transition. When using S3 Standard, lifecycle transitions are actually free, but there is a charge associated with lifecycle transitions when using an S3 Glacier storage class see here.

The exact charge to use S3 Lifecycle Rules for this delete operation will depend on how many objects we're storing in S3 Standard vs. S3 Glacier, and you may also want to consider that S3 Glacier has a minimum storage duration that we will be billed for, assuming there are newly stored objects within this storage class.