I run a SAN and want to charge users for its usage, what method should I use?
In short, because of the complexity of a SAN environment, charging on raw storage is out of scope. In a SAN you seldom know what the real raw storage is that you provide. Trying to figure it out adds a remarkable amount of additional work -- which also means cost -- on your shoulders and decreases the flexibility or prevents you from calculating the cost exactly.
In a SAN environment you should start a storage assessment to find out the cost for operating. With this data at hand, you can start to provide storage services that are based on certain storage classes -- which in turn reflects different levels of availability, accessibility, performance and recoverability. Use these four basic requirements and build a matrix where a certain storage class (your products) has well-defined availability, accessibility, performance and recoverability. Make sure you communicate to your customers how an increase of either of these basic requirements will increase the cost to operate and therefore increase their price.