The price of cloud services is always dependent on how you use them. Details of the billing model can make a difference between something that's a great deal and something that isn't.

GAE is scalable precisely because it gives you few choices. It just takes one architectural decision and bang you can't scale.

What's nice (and naughty) about AWS is that it makes few choices for you. You can use Linux when you like and use Windows when you like. You can usually move legacy systems into AWS. Of course, since it lets you do anything, you're free to build something as complex as you like and you will probably do so until the complexity has you foaming at the mouth.