James Serra links to the Azure Total Cost of Ownership calculator:
For a long time clients would ask me how to determine the cost savings by migrating their applications and databases to Azure. I never had a good answer until now: The Total Cost of Ownership (TCO) Calculator. Now in preview, just provide a brief description of your on-premises environment to get an instant estimate of the cost savings you can realize by migrating your application workloads to Microsoft Azure
I think this is a useful start, though a big part of the value I see in Azure is moving certain things from IaaS to PaaS, like getting rid of the web servers and going to Azure Websites.