Here are some policies of the Google App Marketplace Program:
The Google Apps Marketplace Program Policies play an important role in maintaining a positive experience for Google Apps vendors and users. These policies also help us curb abuses that threaten our ability to provide this service. We ask that everyone abide by these standards so that we can all enjoy the Google Apps Marketplace. Violation of these policies may result in the denial of Google Apps Marketplace access, disabling of your application, removal of your listings from the Google Apps Marketplace, being blacklisted from uploading future listings, or deletion of your Google Account. These policies may change at any time, so please check back here from time to time.The rest of it is pretty much legalese boilerplate that is no different from what the App Store asks of you as a developer i.e. no illegal/objectionable/offensive/infringing material.
BTW to get listed as an app vendor, you have to pay a non-refundable US$100, about the same as being on the iPhone Developer Program. Actually it's worse than the Apple App Store cut.
In general, we ask to share revenue on the total amount your customer ends up paying you for your application and related add-ons. Exactly which revenue is subject to revenue share?
1. Revenue from software installed into Google Apps, whenever they begin paying you (e.g. after any free period).
2. If you charge on a recurring basis, we ask that you share revenue on a recurring basis.
3. Revenue from items that are priced per unit of usage (e.g. per GB, per minute, etc)
4. Revenue on upgrades to your product, including:
1. Plan upgrades, like Standard to Premium2. Additional users3. Feature upgrades, like additional GB of storage
Note that these are additional fees above what you already pay for when using Google App Engine which has already factored the computation/storage costs for the application instance based on usage. In essence this policy states clearly that you are allowing Google to stiff you twice. And you have no avenue for objection because, according to the opening clause, you must abide by the policy or your app might be pulled from the Marketplace.
So what is exempt from "revenue" share? One thing jumps out. Ad revenue. But if you go down this path, you are playing right into Google's game because you have very little control over how and what ads are going to be shown in your app. It might very well be an ad from a competitor.
So how is it that the Google App Marketplace and thus by induction Google itself any more "free" as compared to iPhone as a platform?