How can I calculate how much a high volume site would save with cloud computing Vs. traditional hosting?
Amazon has a cost calculator to help you estimate these things: http://calculator.s3.amazonaws.com/calc5.html
Roll 3d6 for savings, and 1d20 as a saving throw against not actually saving anything.
Or, to be a little less facetious, there's no guarantee that you'll save anything using EC2. It depends on your IO, availability, and traffic profiles. Be careful not to underestimate the hard-to-estimate bits in EC2's cost calculator -- do you know how many IOPS you do to persistent storage each month? I'll bet it's more than you think.