Search engine giant Google, has come up with a pricing model for App Engine. The approach used is similar to AdWords. A user has to set a daily budget and if his/her application exceeds that limit, the quota becomes free for the rest of the day. Further charges are distributed between processing, storage and bandwidth.
According to Google’s App Engine Blog:
You’ll be able to buy capacity based on a daily budget for your app, similar to the way AdWords spending works. You’ll have fine-grained control over this daily budget so you can apply it across CPU, network bandwidth, disk storage, and email as you see fit. You’ll only pay for the resources your app actually uses, not to exceed the budget you set.