Ask HN: How to estimate the resource usage when signing up for cloud services?

While signing up, almost all the cloud hosting providers(Amazon, Rackspace, etc) ask you to provide the amount of resources your site will consume so that they will bill you accordingly. But the problem is how to measure the site usage upfront? If you are already running a site and migrating it to cloud, you would have some metrics in hand. But for a startup, how to estimate these metrics? Guesstimate? Is there any tools that can be used to profile the app usage locally and then convert the metrics to cloud?