Currently, app creators for the Google Marketplace pocket all the revenue from a sale. This may soon change, as Google is planning to introduce commission rates within the Marketplace.
The Google Marketplace was launched six months ago. The plan to introduce 20% commission rates was stated from early on.
The billing API for Google Apps Marketplace will be based on Google Checkout, which should help boost sales for Google's eCommerce system. The 20% commission rate will apply to the total revenue generated by the Marketplace, not just the initial sale. Tracking the varying transaction and billing approaches of it's many vendors certainly will prove to be a challenge.