EXPERT RESPONSE
To address the first part of your question, the answer is yes: There are TCO and ROI models that can be used. The problem is, depending on what the vendor who makes the tools available is trying to sell, the tools will usually tell you the vendor's product is the best "bang for your buck".
The first question you must ask yourself is "why am I contemplating backup-to-disk?" Is it to solve a business problem or is it to cut on costs? Most organizations that are doing backup-to-disk do so to enable faster restores, greater number of mount points, reduced backup data interleaving or dispersion. Not many do it because it's cheaper (although some vendors would argue that point).
However, there is a way to completely eliminate vaulting-to-tape by the way of what is sometimes referred to as electronic vaulting. This can be achieved with the backup software if it supports that feature or by replication between disk arrays. This method completely eliminates tape media handling, but can be very costly due to bandwidth requirements. This is why few companies do it at the moment and the most common implementation of disk-to-disk backups still use tape for off-site vaulting.
Read W. Curtis Preston's answer to this question.
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