When you take a backup, it doesn't use any storage. Whatever you were paying for storage before the backup is the same as you'll pay for storage after the backup. Once you start modifying your data, those differences will cost you money. The backup doesn't directly cost anything, but the fact that you have a backup causes differences to accumulate in the future, so your backup indirectly costs you those differences.
If you were to take two backups and make no changes between the two, the first backup would cost you nothing because it would use no storage.