About where do you usually hit the limit for free usage? Are we talking hundreds of daily visitors? Thousands? Tens of thousands? I know this varies heavily depending on your code, but does anyone have a good rule of thumb?
The times we are charged tend to be for burst traffic when something gets picked up on Digg or Reddit. Recently, one of our blog posts was featured on Digg, Reddit, Slashdot, etc. at the same time and Google dutifully served up a couple hundred thousand visits over the course of a few days. They charged us about a nickel! Granted everything is nicely cached so we were not doing much heavy processing.
It definitely depends on your web app. If you can cache it, App Engine is essentially free. If you are doing something CPU intensive for every visitor, it's going to be expensive.
http://code.google.com/appengine/docs/python/images/usingima...