"On Google App Engine, you're only paying for the resources you use, unlike Azure where you pay for any running compute instances even if your application is unused," Valdez said.

He said also that Google App Engine is easier to use and more scalable than Azure.

"App Engine has completely transparent auto-scaling, with Azure you'd need to correctly provision and tear down instances as demand fluctuates&and this can be real headache," Valdez said. "From the database [datastore] perspective, the business edition of SQL Azure appears to max out at 10GB, unlike App Engine, which has no data limitations. This can be a real problem for even small-scale applications."

Meanwhile, an Amazon.com spokeswoman said that the company's prices are "intended to give users the most flexibility possible in building their applications." She invited developers to use to check the cost of running services such as its S3 storage or EC2 application-hosting service.

While Microsoft is moving their apps to Azure, Ketkar said, it also believes it can win over developers who have already moved to rival cloud services.