You can even buy a fire-resistant safe with USB connectivity, so you can leave an external hard drive in the safe and still back up to it regularly. The ones I've seen are still pretty pricey (~$500), but they avoid the potential for "I remembered to back up, but I forgot to lock the drive back in the safe, and then the house burned down".
Most document safes are rated to keep paper intact for a limited time, when exposed to house-fire temperatures and wattages.
Make sure when you buy a fire safe that it is 'media rated', not all of them are, and the internal temp of an ordinary fire safe will melt cds and backup tapes.
