Nowadays you can buy a 1 TB harddrive for less than 100 euro's . So I build myself a 4 TB NAS box, which is already 50% full. However, although it is to some degree fault-tollerant by using RAID 6, one mistake or catastrophic hardware faillure and all data is lost.
And that's where the 'problem' start. For every € spend on storage, you may need another € to secure that storage.
You can choose to take and accept the risk outlined earlier and not to make backups. However, if you do want to make backups of terrabytes of storage, how are you going to pull that off without too much cost?
In my opinion, the only reliable and usable solution is to build a second NAS box and sync the two. Ideally, both machines reside at different locations, but hey, I'm talking about a home solution, not a professional environment. Although you might ask why on earth you need 4 TB of space at home in the first place.
Anyway, the point I'm trying to make is that although storage in itself is cheap, if you want to keep all that data safe, storage is far more expensive than you might think.