Tangential question: What's the absolute cheapest storage per byte right now for someone who doesn't care about speed? Is it like a 4TB HDD? Any specific examples would be appreciated.
Yes. It's what Amazon Glacier runs on: https://en.wikipedia.org/wiki/Amazon_Glacier#Storage
The Backblaze blog is also a great place to learn about a large storage deployment: https://www.backblaze.com/blog/hard-drive-benchmark-stats-20...
If all you need are two hard disks, go for it.
Since then the price went up by 20% for reasons that are beyond me.