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To figure out if cloud storage is really cheaper, you first have to define "cheap." Is it a better value? Is it lower cost? The process to figure this out might be comparable to that of leasing a car, which can sometimes be cheaper than buying one depending on how many miles you will drive it and how long you intend to keep it. You also need to consider your cash flow and current methods of evaluation.
There are values and economies of scale with cloud storage, as well as the application benefit to consider. If you're just looking at cloud storage because of its cheaper cost, you may not be taking into account some of the other benefits of cloud storage.
Notice I didn't say "costs," I said "benefits," which refers to having somebody else taking care of your data and ensuring it's protected, and providing the infrastructure or leveraging those economies of scale that may in fact complement what you're doing. In other words, cloud is another tier; it's another resource.
However, determining the cost of cloud storage can be tricky; there are upfront costs to consider in relation to the low cost per gigabyte to store data, but you also need to know how your current costs compare. Do you know your own cost for general-purpose, database or virtual machine storage? Do you know your own costs for protecting to a given level of service? If you don't, how do you know if the cloud will be a cheaper proposition or not?
You not only need some insight into your own environment, but you have to understand the costs within the cloud -- the upfront costs and the additional fees.
The big providers document what those fees are, the limits on I/O, the durability and availability, and all the other caveats. They don't hide it, but you usually need to go to the site and find it.
In some cases, the cost of cloud storage can be cheaper upfront, but you have to look at it at different sizes and scales, and ultimately from the standpoint of what the business benefit to your environment will be. There should be a benefit to using the cloud beyond simply the cost per month.
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