If you wanted a full clone of the drives then you could use something like Macrium reflect to backup the drive as a .img file, it will do full and differential backups in the free version.
Running this on every machine to a central storage unit (NAS) or to a plugged in HDD would work, this could be either individual or a monster HDD. You could partition the drive into the relevant sections if you used just one drive.
You'd need to buy a large enough drive though, and it'd need to be at least the size of all your drives put together to ensure you could keep at least one full copy of everything. If you wanted to keep versions then a larger drive would be in order.
For me a large enough HDD wasn't affordable, I worked out how much data I'd have in about a years time and got a drive that size, luckily that was affordable. i.e I have a storage cap of 5.44TB, but only 1.87TB used, that's taken 3 years to get so I got a 3TB HDD as by next year I can upgrade.
I can't think of a cheaper way of backing up the data. Remember that it's suggested to have an offsite copy too, although not necessary. And if you don't version you won't protect yourself from deleted files in the long run.
Anthony.
Macrium also offers a paid version that you can backup files and folders, might be another option if you didn't need to clone the OS. Software like CrashPlan offers a free backup to local HDD (int/ext), you could plug that into a machine and backup the rest via the network, or just plug into each machine when you want a backup. Crashplan will recognise the drive being plugged in a begin the backup, reducing user input.
edit: Macrium Reflect or Acronos True image could be used.