I have to disagree. If you have data that does not always need to be online and have fast R/W access, DVDs are a *lot* cheaper than HDD.
If a 400GB hard drive is $100, working out at 25c per GB, high quality DVDR media (Taiyo Yuden/Verbatim) is less than 40c each, working out at about 9c per gigabyte, or 64% cheaper. And that's with UK prices on DVDRs. Comparing to UK hard drive prices, you're looking at 70-75% cheaper.
But I'm also taking into account the hours (and days) of actually sitting there inserting a blank CD, futzing with Nero or Roxio or whater, burning the disc, and then labeling it. Unless you have one of those $15,000 automated DVD robotic towers that will burn a stack of 50 discs. Or you have a trained monkey. If you include your time and hassle, is it REALLY 9c ? Or if you amortize the $15000 tower or food&vet bills for the pet monkey into the backup costs, is it still 9c per GB? :-)
The key here is to not leave the extra backup harddrive(s) running all the time. So 99% of the time, they are disconnected from the computer. Think of the 400GB as a gigantic compact flash card. If you do this, harddrives don't fail as often. I think I've upgraded and migrated data through more than 80 hard drives in the last 10 years. I've only had 2 failures: the infamous 75GB IBM 75GXP and a Maxtor 200GB that failed fresh right out of the box. No other failures ever, regardless of brand.
I currently have 20+ terabytes of personal data. I've researched many ways of backing it up. If you have hundreds of gigabytes to "archive" or backup, I can't see that being done via a stack of DVDs or BluRay. I still stand by my statement that backing up to extra hard drives is the most cost effective and hassle free way to do it.
If a manufacturer comes out with holographic optical disc with terabyte capacity, then I'll probably go that route if media costs aren't too high.
Now, if anyone here has actually tried to backing up a half terabyte of data on to a stack of DVDs, I'd be interested in hearing about the system setup and workflow to minimize the hassle. Maybe I missed something.