Yup, at minimum you'd need another NAS off site (say, at a friend's house) so you always have a copy. I have my NAS in a mirror to ensure I can recover if a drive fails, so that would mean 4x the cost of whatever storage size you need if you want to ensure your data stays safe.

Just some quick math, a WD Red Plus 10TB drive costs $190. So that's $19/TB, and they have a 3-year warranty (used to be 5), so let's assume they last 3-5 years. I need four drives minimum (two sets of mirrors), though if I have a lot of data and drives, that'll go down (e.g. if I can use RAID 5 or RAID 6, I need less parity):

19 * 4 / 3 = $25/year
19 * 4 / 5 = $15/year

So $15-25/year, or $1.5-2/month, which comes with a few caveats:

• can't easily expand storage (e.g. can't just add 1TB), so need to overbuy; I have 8TB storage, but use less than half of that, so probably double the above cost
• ignores PC costs, which can be hundreds every few years ($5-10/month) to replace aging components (esp PSU and RAM),; ideally get ECC RAM to reduce risk of bit rot
• ignores electricity - assuming 100W, running 24/7, and $0.12/KWh, that's ~$9/month

There is a crossover point at which self-hosting is cheaper, but if you only need 1-4TB of storage, something like Backblaze is probably cheaper. But as you get bigger, the NAS looks more attractive.

Alright, so you're telling me I should invest year 2 of a Backblaze sub in having a second NAS set up off-site?

That still pays off pretty quickly.

Alright, look, in all honesty, what you want is to mix and match. I'm not gonna sit here and break down my entire data storage strategy, but you do want multiple solutions in parallel. The point of NAS is that you get mass storage you fully control, so it's most cost effective for things that are huge and that you want on hand. Like, say, backing up your physical media or your digital purchases. That's pretty close to good enough, since you probably retain access to your disks or your subscriptions and the NAS acts as a backup anyway.

Sure, despite my UPS protection and data redundancies my NAS could be nuked froom orbit and all of the stuff in it could die. And Google Drive could at some point decide to just poof six months of user data into the ether. What you really want is two separate backup solutions. Just don't go nuts and acknowledge that your source media is also a copy of your media. This is an expensive rabbit hole. I still wouldn't pay thousands of dollars a year for somebody else to run my mass storage. It's more cost effective to keep the huge stuff in a NAS and perhaps a backup in a DAS box somewhere. Unless you're curating a museum or doing life and death research that's probably more than enough security for your media files.