Thanks for the clear description. In your case, 50 GB will be billed.
Check https://aws.amazon.com/ebs/pricing/ and https://aws.amazon.com/premiumsupport/knowledge-center/ebs-snapshot-billing/ for more information.
Example 7 – EBS Snapshots Snapshot storage is based on the amount of space your data consumes in Amazon Simple Storage Service (Amazon S3). Because Amazon EBS does not save empty blocks, it is likely that the snapshot size will be considerably less than your volume size.
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