Shameless plug: I built a backup service[1] just for Borg and the price per TB on the large plan is $5/TB. Not as cheap as "cold storage", but still better than rsync.net and the same as B2.

Also worth pointing out that my storage is calculated after compression and deduplication. So depending on the data a Borg backup can be much smaller than the actual data.

1: https://www.borgbase.com