The good thing about appengine is that they bill you for cpu/hour used, you only pay for what you really use.
I also considered appengine as an option as back-end when i was thinking about a multiplayer android game a few months ago but the thing that bothered me was their limit on api request/s that at the moment is at 30K/minute. It didn't seem enough for an app as the jay's one that needs nearly 10 requests/s.
Guess i was wrong.