Right off I don't recall, but when working under a project we get all the figures for this. Yes cost for the storage size itself goes down but there is the cost of power consumption, physical space, non-disk hardware infrastructure (including cool systems such as large AC units and rack), software licenses, and support personel, plus your DR infrastructure that often are overlooked as a user I may not pay directly for them, but the corporation I work for sure does and there are people who track these things to make sure things are consolidated where and as much as possible.