How can you monetize a Google App Engine site?

Would you be able to charge users for parts of your service that are provided via Google App Engine?

1) Is it technically possible to build in a payment method to accounts? (I'm pretty sure it would be if you have a free product and ask your users to pay for additional services and features.)

2) Is it allowed under the Google App Engine terms and services?

3) Will end users who are used to having everything free from Google pay for a service that they use their "free" Google login to use?