WiebeTech discusses why it makes more sense to use hard drives for backups than CDs or DVDs.
How much does it cost to burn to CDs?
For a spindle of 100 CDs you're probably going to pay from $10 to $30 at the corner computer outlet store - depending upon if you buy a name brand or not.
Let's say your data is important, so you go ahead and buy a $30, 100 disc spindle of 80 minute, 700MB CDs. You just bought 70GB of storage space for $30 - so, cost translates to about 43 cents per Gigabyte. Sounds great, right?
Let's assume that you're not going to be able to optimally fill 700MB each disc burn - let's say you're using these as archive discs, and your average is actually pretty high, and you manage to fit 650MB per disc - so the cost is now at 46 cents per Gigabyte. It still sounds OK up to this point...