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.