Amazon will pay perhaps $50 for a 10TB disk, 10x it to cover for redundancy and the servers and datacenters to put the disks in, and you're looking at "only" 16 mil savings for an exabyte.
Let's say a 100 exabyte, which is only 100 times larger than Backblaze. And Amazon pay $100 per 10TB, it is unrealistically low, but just to round down the number nicely . ( It is not $50 for 10TB, part of the reason why HN dont understand Supply Chains and BOM cost. )
On pure storage that is 100,000,000TB. That is 1 billion dollar already excluding any redundancy required by S3.
Did some searches but came up with some guesses like https://maisonbisson.com/post/how-big-is-s3/ but they are dated and still just guesses.