Introduction
Someone posted on a Facebook group looking for the cheapest means for cold storage backups. I did some research and collected some data.
Response
Tapes
All depends, if you have a petabyte or half a petabyte it might work. It might be cheaper to just sync data to another data center. LTO8/9 are 12/18TB uncompressed.
But you have to buy new tape drives every 5 years as the technology changes and data gets bigger. If you have a tape library that has four drives, it can be costly.
You also have to rehydrate, sending tapes back and forth and having backup software manage them. You also need to replace tapes due to lifespan or duplicate tapes for redundancy (they’re not 100% reliable).
For a small setup, you could look at this, which offers to stage. I haven’t used it but looks cool https://eshop.macsales.com/shop/owc-mercury-pro-lto
You’ll have a massive upfront cost for tape, the drive being the most expensive, then an appropriate HBA which is usually pretty cheap, and then the tapes, which are also relatively inexpensive.
But what about the storage of the tapes? Do you ship them to a friend? What software are you going to use for backups? There are lots of caveats with tape, even with LTFS https://getprostorage.com/blog/lto-ltfs-archiving/
SSDs + Safety Deposit Box
You could get a safety deposit box, buy 4x8TB SSD’s M.2 and plop them into this bad boy and get 32GB RAW or less in a software RAID.
https://www.storagereview.com/…/qnap-tbs-464-mini-all…
You could buy two or even a spinning disk QNAP and rehydrate every month.
The only issue is M2 SSD’s are expensive, you’d want a SATA 8TB for around $900 pop. and grab this little guy https://www.storagereview.com/…/synology-diskstation…
Or you could just buy 2TB SSDs and use a docking station like this https://www.amazon.com/…/ref=cm_sw_r_apan_glt_fabc…
Use SSDs like tapes. Just keep an eye on https://diskprices.com/ for the cheapest per GB SSDs. The cheapest SSD out there is the SAMSUNG 870 QVO 4TB.
You could put the SSDs into an electrostatic bag with a dry pack and seal it 
Online Storage
Backblaze B2 (Per TB)
At USD $5/month/TB this is pretty affordable if you have over 20TB per month you can reach out for reserve capacity which requires time commitments.
https://www.backblaze.com/…/reserve-capacity-storage.html
Backblaze Personal Backup (Unlimited)
At USD $5/month if you can use Backblaze Personal Backup, you can back up an unlimited amount of data. The operating system would just need to be able to see the data. This doesn’t come with versioning.
Backblaze Largest Personal Backup (2018)
I saw this thread on Lowend Box about the largest personal backup at Backblaze on the Personal Plan that is $5/month. Granted this is data from 2018.
https://news.ycombinator.com/item?id=20998010
Here’s a screenshot of the post.
Here’s the screenshot from imgur.
Here’s the original image, in case the one from Imgur get’s taken down.
S3 Glacier
Cost USD $3/month/TB, and actually cold storage.
Mega.nz
Cost EUR €1.56/month/TB, not cold storage.
Wasabi
Cost USD $5.99 TB/month, not cold storage.
OVH Cloud Storage
Cost USD %$9.5 TB/month, not cold storage.
Conclusion
There really isn’t much of a conclusion, the cheapest solution is Backblaze but you can’t backup NAS devices. Mega.nz seems to be the cheapest.