As others have said, it’s a complicated question, but if you have the resources/wherewithal to run Ceph but don’t want to deal with co-location, you can get a bunch of storage servers from Hetzner and get a much better grasp on cost over S3.
For example, at 10PB with every object duplicated twice (so 20 PB raw storage), you’d need ~90 of their SX293[1] boxes, coming out to around €30k/mo. This doesn’t include time to configure/maintain on your end, but it does cover any costs associated with drive replacement for failure.
I’ve done similar setups for cheap video storage & CDN origin systems before, and it’s worked fairly well if you’re cost conscious.
[1] https://www.hetzner.com/dedicated-rootserver/sx293/configura...