This is probably the most expensive option. Buy the hard drives you plan on backing up data to and then use a secure hierarchical file management tool like Cloud Storage Server:

https://github.com/cubiclesoft...

You can use the recommended Cloud Backup software or use whatever reliable backup software solution you prefer. Put all of your data onto the drive(s) first to save yourself the pain of the initial upload and then, when you are ready, drive them over to your friend's or relative's house and hook them up to their network. Then push regular incremental changes at night. Be sure to reciprocate and offer them the same service.

This approach backs up data off-site but still in the same town/city, meaning your data is fully recoverable within hours of most major data loss disasters. The only exceptions to this that I can think of are area flooding and nuclear attack. However, someone might prefer this option over dumping their data to some unknown data center or having it sit on a complete stranger's system. At the very least, it can be another place to stash data, is a fixed purchase, and you know exactly what "unlimited" means because you can't exceed the amount of physical disk storage that you purchased.

This approach is completely custom as you are basically building your own cloud storage hosting service in someone else's house.

I recently ran into Sia and Storj. Both of those are relatively new cryptocurrency blockchain based backup systems. If you are into that sort of thing. Basically, you obtain bitcoin, convert it into Sia or Storj currency and then use the respective software to form contracts with hosts around the world to host your (encrypted) data. What turned me off to these options are two things: Becoming a host requires 99.5% uptime, the host must place collateral into the system to begin hosting data, and requires running a Linux daemon whose source code is of unknown quality (because I can't be bothered to read that much source code for network-enabled software - sorry). Beyond the self-hosting requirements, the requirement to start up the client every few days and let it settle was a complete showstopper. Also, the lack of a simple programmatic API for sending and receiving data externally was the other major problem: I can't easily point my own software at the system and make it work for me. But, hey, the estimated $2 per TB per month could be right up your alley. So I figured I'd mention these options.

I'm rather shocked that no one has mentioned it yet but check out OpenDrive. If you need a backup tool that supports OpenDrive, here's one that I actively maintain: https://github.com/cubiclesoft... It looks like OpenDrive recently lowered their prices from $13/month and added more options. For $9.95/month or $99/year, you can back up "unlimited" data. I use their service. Faded out in one spot in their per-user configuration, I once saw a user quota box with a faded out "1,024 TB". In my experience, both OpenDrive and Amazon Cloud Drive are extremely flaky cloud backup services that like to completely break in the middle of a backup for inexplicable reasons. The services could also vanish at any time. Amazon's service certainly looks to be headed that way. You can probably blame the DataHoarders subreddit for the service shutdowns.