CD-R drives are dirt cheap under $20), especially from mail order sources like geeks.com. If your data volume is small, CD-R will give you the most portability but if your backup files exceed 700mb, then DVR has come down enough in price to be a consideration. How does that $15 - $20 range compare with what Office Max is quoting you?
About 1/3 to 1/2. Of course, I prefer buying locally. If it croaks (under warranty) I can get it replaced the same day.