There was a good blog post about the real cost of storage, but I can’t find it now.

The gist was that to store 1TB of data somewhat reliably, you probably need at least:

• mirrored main storage 2TB
• frequent/local backup space, also at least mirrored disks 2TB + more if using a versioned backup system
• remote / cold storage backup space about the same as the frequent backups

Which amounts to something like 6TB of disk for 1TB of actual data. In real life you’d probably use some other level of RAID, at least for larger amounts so it’s perhaps not as harsh, and compression can reduce the required backup space too.

I have around 130G of data in Nextcloud, and the off-site borg repo for it is about 180G. Then there’s local backups on a mirrored HDD, with the ZFS snapshots that are not yet pruned that’s maybe 200G of raw disk space. So 130G becomes 510G in my setup.