I like the idea, it's just a bit difficult to use for what I'm getting out of it. Simple things like making clear what the "number of days" slider means (it didn't take long to figure it out, but I had to use the context to draw my conclusion). I'm used to seeing my instances in the typical "t1.micro, m1.small, m1.medium, etc" format, so I was a bit thrown off by the "Instance Category" dropdown.

It would also be nice to have values on the x axis of the graph so I can see exactly when certain thresholds are crossed - as of right now I'm just assuming that the max x-value is my specified days * 24.

Again, cool idea, but I'd be more inclined to hit up the AWS Cost Calculator (http://calculator.s3.amazonaws.com/calc5.html). It's ugly, but it allows me to quickly add many resources from all of the AWS services for a total monthly cost.