RDS snapshot will be taken based on the size of your database and not based on the total storage size of your database. First snapshot will be full backup and subsequent snapshot will be incremental.
For example, if your database total storage size is 20 GB and you have the data of only 10 GB, first snapshot will be 10 GB and the next snapshot will be based on what changes occurred since last backup (it can be 1 GB/2GB/3GB, depends on the changes).
There is no additional charge for backup storage up to 100% of your total database storage for the region. In your case this means you get 20GiB/month of backup storage at no additional cost.
Anything above this is charged at $0.095 per GiB/month. The size of these backups will vary depending on the storage usage, so there will be some variation each time you take the backup depending on the change rate and total storage consumed.
Without knowing your storage consumption and change rate it's extremely difficult to provide an estimate of cost.
Thank you so much Andrew_R and Vetrivel.
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