Estimating your costs in the cloud may seem like a scary proposition. There are many factors to consider, and overlooking one can paint a misleading picture. To help people understand some of the less obvious factors, I've started a blog series called Hidden Costs in the Cloud that focuses on the Windows Azure Platform. Here are the links to the first two parts of the series:
Hidden Costs in the Cloud, Part 1: Driving the Gremlins Out of Your Windows Azure Billing
Hidden Costs in the Cloud, Part 2: Windows Azure Bandwidth Charges
Also check out some real world input by developers on Azure costs.