Have you considered a 'cloud' backup option? the Data is stored on someone else's servers, which also gives you disaster recovery.
I use crashplan and have about 3.8TB stashed away for 10 quid a month or so.
You can also use an off the shelf (including free) backup from each machine to a central one (just stick more hard disks in one, they're cheap). and then back that up to the cloud.
It may take a while to catch up with your current storage, but with modern internet connections, it keeps pretty up to date.
As Stuart says, it's not entirely simple, as there are trade offs, but cloud is a useful option to consider.
Iain