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.
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.
On pure storage that is 100,000,000TB. That is 1 billion dollar already excluding any redundancy required by S3.