How does the cost of hosting on Google App Engine compare to hosting on more classical servers for a high traffic website?
What's the answer for a website with 1000 page views per minute and how do the costs compare at 10000 page views per minute?
So it sounds like you're concerned that Google App Engine will work and scale for you, but you're going to get to a point where you exceed their capacity to host you without being costly. Is that right?
I think the thing to do here is to have an "exit strategy" for your App. I think you need this regardless of whether you exceed Google's limits or not. What if Google decides App Engine is a money loser? What if it stops supporting the APIs you want to use?
Check out AppScale, which is intended to be an open source version of Google App Engine for what might end up as your fallback should you outgrow GAE's limits.
I was going to post this as a comment but just since you wrote "My website isn't built yet", please take a look at the limits for a free account:
Per day Per minute Requests 1.3m requests 7,400 requests/minute Outgoing Bandwidth 1 gb 56 mb/minute Incoming Bandwidth 1 gb 56 mb/minute CPU Time 6.5 CPU-hours 16 CPU-seconds/minute
If you think you will use a lot more than that, it's one serious web site.