I back my sites up daily using cPanel (argh), even though all material are on mirrored disks (redundancy).
Using cPanel/WHM gives me the option to backup off site at
Amazon S3. Their free tier includes 15GB of data. However, it does include some restrictions such as data in and data out. With that being said, a small network of sites could be either free or relatively cheap (less than $10) to back up monthly. All you require is data in for the backup (free). Nevertheless, if you do need to expand, data out is fairly on the low side as well at 12 cents per GB up to 10 TB after your first GB (as backups generally include material that could get lost and wind up losing you money).
Another option if you don't have root access; rather a reseller or cPanel account, is to create a site backup and download it. A typical full backup for a minisite is anywhere from 50-120mb (probably because of catch-all's that I've setup).
I can download this directly from my cPanel account and upload it to my Google Drive account that also includes 15GB of free data.
In essence, if I wanted to store 30 GB of data, I could.
Though, I could have a hardware fail over (mirroring), backing up to S3, downloading and making a personal backup on my home computer(s) and cloud drives as well as using Google Drive for 15 GB of backup space.
I try to download or transfer out a new copy of my backups once every 2 or 3 days as I see that's when they normally operate and not much data would be lost in a catastrophic failure. I do not keep the backups on the same server, even with a fail over. I do keep backups on another server in the same data center, but even then, I am cautious to move them out.
My logic behind my backup procedure is that my websites make me money. If I were to lose data, I am losing money until I can find a solution to get it back (send my drives off to get the data recovered, if they can, etc. which would cost me even more).
I hope that helped answer your questions.