Azure vs Appharbor vs Amazon EC2 [closed]
AppHarbor will definitely get the job done.
Azure is also a PaaS (with lots of infrastructure features), but there's no add-on program so you're stuck with whatever services Microsoft decides to offer (or you have to install, configure and maintain them on your own VM-role instances). And as you mention, Azure gets expensive quickly.
AWS Elastic Beanstalk also has .NET support now, giving it some PaaS-features. The deployment-model is not very sophisticated though (and neither is Azure's): You have to have a Visual Studio plugin create packages that are then pushed and deployed whereas AppHarbor integrates with GitHub, Bitbucket, Codeplex, etc., runs your unit tests, sends build notifications and lots more.
(Full disclosure: I'm co-founder of AppHarbor)