Cost of storage

I'm trying to put together a roadmap for our Enterprise Storage group, and part of it is trying to show the expected cost of storage over time for different scenarios. Unfortunately I'm having a terrible time trying to gather information on the price of enterprise class storage, and how it has changed over the years. It is obviously getting cheaper as time goes on, but how much? Historically we only buy storage every 3-4 years, so I have a grand total of 2 data points to run with. Not too helpful...