Developers have been quick to take advantage of in-app billing which has just arrived for the Android market.
The ability to sell extra features within games and subscriptions to magazines is an established feature of Apple's iTunes store but until now the Android Market lacked equivalent functionality. In-app purchasing has now finally arrived and some games apps have already started to take advantage of this source of extra revenue. Developers can now charge users for extra levels within games or to buy virtual gifts such as bunches of flowers or champagne for a virtual friend.
As the Google blog states:
In-app Billing gives you more ways to monetize your apps with try-and-buy, virtual goods, upgrades, and other billing models.
For developers who haven't used in-app billing before there is a useful discussion of its architecture on the Android Developer site.
It's not just developers who will be making extra money, as Google will be taking the standard 30 percent cut of all in-app purchases.
For detailed information:
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Android apps on the rise
The remarkable spread of Android
Android Apps Marketing (book review)
Introduction to Android 2 application development