multiple direct attached storage
Buy two or three external drives (about $80 per 2 TB drive these days?) keeping one attached to the machine at any given time. Once a week you replace the attached drive with the oldest of the three and move the now newest drive to a safe secondary location.
You also don't need to backup the entire drive since some of that will be osX itself which is easily restored from the original install disk. You really only need to point Time Machine at your user data. Applications are a bit of a grey area as they should be recoverable from original install media but time concerns may justify having them in the backup image.
That would be my thinking on it anyhow. With my own *nix systems, I backup the user home directory and any one-off storage directories outside of that (eg. VMs and related ISO images fall outside of the user home directories on my system). Since my reinstall from bare metal can be done in about an hour, the recovery process would be to do a fresh install of the OS then spend the second hour restoring those non-OS directories and user home directories. osX recovery would probably work the same though thankfully, our backups have been more about recovering accidentily deleted files rather than full system restores.
