Why pay that much for a tape backup system that runs 40GB tapes when he doesn't need nearly that much?

In my experience, the amount of data needing to be backed up will increase exponentially as time goes on. Investing in that kind of headroom is not illogical.

Besides, like we were saying, you can append to tapes. So if he's only backing up 2gb, then one tape can contain 20 full backups.