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Can you help me with my capacity planning?

I would like to scale a web application to the Cloud and wanted to know if anyone had any experience calculating costs and could tell me how I would go about that. I have NO experience with cloud services at all.

Currently my production environment consists of two web servers and one database server.

If the application continues on a linear growth path, eventually, I want to scale to the cloud to avoid any more long term commitments to extra hardware.

I want to be able to create a similar environment I have now as a baseline. Have this as my fixed cost that I will always have.

I also want to calculate my variable costs that will increase with more users or bandwidth.

I don't have a preferred cloud vendor. Amazon, Rackspace, Terremark or any other is fine as long as understand how to calculate my fixed and variable costs.