I'm sorry but I don't understand the question, but I'll try anyway. The ROI for SAN is built around connection flexibility to storage resources. Measurements for utilized storage (as opposed to storage that is trapped by DAS systems).
ROI also comes from the time it takes to add storage capacity to a given system. If it's parallel SCSI, the system has to come down for a time while the new storage is positioned and connected. If it's a SAN, the new storage can be placed anywhere in the room (or within a campus) and the storage can be logically attached to the system through a FC login. Downtime is minimal and can be avoided all together with the right combination of OS and SAN configuration.
There can also be real ROI from centralizing backups and calculating the cost of managing tape media distributions.
Hope this helps,
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This was first published in January 2002