I did a lot of searching around on this question about 18 months ago. The least cost backup provider that I found was Amazon Jungle Disk. Google them and you can get the rate per GB of storage and MB of transfer. I really like it because it keeps as many prior versions of files as you specify, it lets you set frequency of backup independently at the file or folder level, and for large files it will upload only the portion of the file that actually changed...so it saves MB of transfer.
We keep everything that needs backing up on our server. In fact, I direct all staff NOT to use their individual PCs for work files. This allows us to just backup one disk. Also, the "Jungle Disk" backup is mappable as a network drive, so restoring a file (or previous version) is as simple as navigating to a mirror folder on another drive, copy, paste.
Good Luck,
John