That must be for SSDs, which are ~$150/tb vs ~$20/tb for HDDs. I expect IA must use HDDs for the bulk of their data.

SSDs are about $400/TB for raw storage. Prices are substantially different for consumer and enterprise spaces, but 4TB disks (Micron, Samsung, etc) tend to be about $1500 and 8TB around $3200. If you factor in redundancy, you're looking at around $1000-1500 per TB of usable storage.

This also isn't a matter of "just go with cheaper consumer disks". If you care at all about data integrity you won't use consumer-grade SSDs.

Or even tape. For offline redundancy it’s still cheaper.

3-way mirrored spinning storage would cost roughly $100/TB. 12TB disks are about $350/ piece these days. There are a whole bunch of possible considerations and alternate scenarios, but this is probably the worst-case.

That does not factor in compression / dedup, which can gain you a substantial amount of savings depending on your work profile; I bet if Archive.org wanted to, they could slash their storage using block-level dedup. Of course that creates potential recoverability headaches, so it's possible they don't.