@wizard $5 per user per month, so $60 per year.
I want a cheap offsite backup for my server, but I'm currently using almost 19TB. A lot of that doesn't need to be backed up, but it'd be nice to have important files backed up somewhere. The platform doesn't really matter as long as the backups are encrypted.
The cheapest option for you would probably be to just buy another nas and put it somewhere at that amount of data. Doing some cursory looking, to store it all in an s3 One Zone Infrequent Access bucket would be about $194, just to have it there. That might be the cheapest option for cloud storage even.
I guess you could buy 20 licenses to 365 Business Basic if you /really wanted/, but that'd result in some fractured data and surely they cap you somehow anyway. OneDrive isn't really a backup storage platform.