Tony you hit the nail on the head. There are a number of variables for storage costs.  Cheapest would be to buy a TB hard drive and store on that but if you're looking at multiple TB's then you need to look at tiered storage, and costs associated such as backups, DR testing ect.  
Areas you need to address -
Speed which has the higher costs, should be the most current information
Volume -reduced costs-start by archiving files that have not been accessed for a year or six months (whatever you're comfortable with)
Archive-lowest cost but longer to retrieve

Here is another link and I cannot vouch for the numbers, they are what the author says they are.

http://blogs.hds.com/david/2013/07/big-data-cost-tierratio.html


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