Matt, you might want to look at DynamoDB, which would at least let you do a cost comparison to an AWS stack.

This tweet from former AppEngine dev Brett Slatkin caught my eye: https://twitter.com/haxor/status/411351463806263296

I still think Dynamo has a ways to catch up in terms ease of use to GAE, but it provides the performance, scalability, and replication like datastore.