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.