I recently bought a spnidle of CDs for 50 AU cents each (cheap) which were rated for 1-24x burning. This was, of course, a bold-faced lie, and the Finder's Disc Burner made a decent stack of coasters proving the fact.
You can, however, burn images straight from Disk Copy, which provides an option to set the speed manually. Really cheap media should work OK after you dial down the speed to 4x or 8x ... so that's what I do for archiving and general storage purposes.
[Editor's note: I have not tried this myself ... and I'll add that you probably shouldn't use the least expensive media you can find for burning the most important files on your machine!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020525095709117