I've had App Engine projects that were on Product Hunt and Hacker News for a day, and the bill for those days were close to 0. (it did exceed the free tier, but not by much)

You probably wouldn't want to run Reddit on App Engine, but anything with medium traffic or decent monetization should work out just fine.

The only other caveat is that it may depend on what the app does and how you utilize the datastore. If you don't consider cost of the datastore when building an app, it could end up being expensive like you mentioned.