Have you considered the cost of the tape loaders? Our loaders cost significantly more over their lifetime than the storage costs of the tapes themselves.
The tradeoffs will be different depending on how many tapes you write and how often you reuse them.
Until I took over backups, and instituted a rotation methodology, the guy prior to me just bought another 60 AIT-3 tapes every month and shipped them off site to Iron Mountain.
Agreed - how-often you re-use tapes (and whether you do) - has a dramatic effect on "system cost" of your backup system.
The tradeoffs will be different depending on how many tapes you write and how often you reuse them.