A hard drive sitting on a shelf is a big question mark. Will it spin-up? Who knows?!

So you deal with this by having a backup. Now you have 2x the cost.

But, unfortunately, you could have two hard drives not spin up. You consider an additional backup, but at this point you're at 3x the cost, so you start to reconsider letting hard drives sit on shelves.

Now you're in RAID country. RAID is not a backup, but by shifting your focus to availability you now have an approach to dealing with the "will it spin-up?" issue by proactively dealing with the problem via monitoring and rebuilds when necessary. You keep several replacement HDDs on hand for this inevitability.

Now, since you know RAID is not a backup, you put a backup system in place. This is additional cost; even more if you go 3-2-1.

So now you've got hard drives, storage arrays, HBAs, backups, and the power all of this consumes. We're starting to get out of cheap territory.

I suppose what I'm trying to say is: If you have a lot of data, hard drives are only cheap if you don't care availability, integrity, and longevity.