If those 20PB are deep archive, the S3 Glacier bill comes out to around $235k/year, which also seems ludicrous: it does not cost six figures a year to maintain your own tape archive. That's the equivalent of a full-time sysadmin (~$150k/year) plus $100k in hardware amortization/overhead.
The real advantage of S3 here is flexibility and ease-of-use. It's trivial to migrate objects between storage classes, and trivial to get efficient access to any S3 object anywhere in the world. Avoiding the headache of rolling this functionality yourself could well be worth $3.6M/year, but if this flexibility is not necessary, I doubt S3 is cheaper in any sense of the word.