Forrester Research Inc. has released a report and an associated analysis tool that offers a methodology for comparing the cost of in-house file storage to the cost of storing files in a public cloud. The report illuminates many variables beyond simple dollars-per-gigabyte base-cost comparisons.

The report runs a sample set of data using mid-range values for numerous variables involved in storing 100 terabytes of data for four years. In that example, compared to local hosting, the cloud reduces the cost by 74%. However, if the cost includes a cloud storage gateway service—comprising a client-side server and software to cache often-used data and manage the integration between local applications and cloud-stored data—the cost reduction drops to 25%.

The parsing of the factors that should be rolled in and the encouragement to evaluate the cost using a firm's own data are key elements of the report. For example, when weighing the cost of internal storage, assumptions should be made about the number of years data will be stored; the number of copies kept for backup and archiving; the actual usable storage capacity of a system; the cost of disaster recovery provisions; and the cost of maintenance and staffing, power and facilities, and periodic data migration as hardware is upgraded.

Among the variables that should be weighed for cloud storage costs, the report highlights data transfer charges, the cost of redundant copies, the cost of a gateway, the likely need to upgrade a wide-area network service and monthly base charges.

Cambridge, Mass.-based Forrester claims independence from clients that sponsor reports, which, in this case, is Amazon. Further, by focusing on methodology rather than offering head-to-head product comparisons—and by offering a customizable spreadsheet for running analysis using a company's own data—Forrester's neutrality appears to be borne out.

On the Forrester Research website, the report and the Excel anaylsis tool are available for $499 each.