The cheapest way to backup your data is to use external devices like CD/DVD or USB drives.
Let us consider the features of the external media currently available:

1. CD-ROM Disks: One of the cheapest backup media available, the CD can store about 700 MB of data. One can easily store data on a CD with the help of CD-writers which are common in most computers available today.
2. DVD-ROM Disks: These are a bit costlier than CDs but considering that one DVD disk can store 4.2 GB of data; the cost per GB is less as compared to CD.
3. USB Drive: Falling memory prices have made these backup media very cost effective. The additional advantage of these drives is that they can be reused many times unlike the CD/DVD. The only drawback is that usually a USB drive is required on the computer which may not be present in all computers.