Considering shifting a service to the cloud but not sure if the business case stacks up? Amazon's newly-launched TCO Calculator for Web Applications gives you a basic idea of how much you could save.
The calculator lets you set a large number of parameters, including the required number of web application servers, database servers and storage requirements, as well as predicted growth rates, whether you want multi-site coverage for added redundancy and where the servers are located. The calculator will generate a basic chart and a detailed report outlining potential cost savings. Amazon says it is working on expanding the calculator to cover other types of application scenarios.
Obviously, the calculator figures and assumptions are entirely Amazon-centric: if you're contemplating using cloud services, you'll want to consider rivals such as Azure or Ninefold as well. Nonetheless it's a useful tool for examining cloud costings.