Basically your question comes down to figuring out what you want to assess. Do you want a snapshot of the storage environment? Do you want to perform a trend analysis? Or do you want to gather as much data as possible, just to have it all? Before you do anything else, you have be clear on the question that you want answered.
Storage assessments aren't trivial tasks that simply deliver some data. Gathering the data is one thing, but interpreting the data is another. To gather the data you can use an already running SRM system or, if you do not have such a system implemented yet, have an external consultant do it for you (this includes the interpretation of the found data as well).
I am not aware of an inexpensive software program that will work in an open systems environment. And there are only few SRM systems that work in an open environment. For example, Sun Microsystems has it's StorEdge Suite (formerly HighGround SRM), IBM integrated the Trellisoft SRM into Tivoli and both of these systems can be used. However, both systems come with at least a 5 figure price tag, which is certainly validated if you intend to implement SRM in your environment.
If you just want to asses your environment and do not want to implement the whole SRM system, it might be best to get outside help.
If you look yourself for other software products that can gather the data you should take the following caveats into account:
This list shows some of the "hidden areas" you should be aware of when deciding what software to use for an assessment.
Hope that helps a bit.