Complete Picture of Android Revenue is Still Difficult to Determine
Google develops the Android operating system and gives it away for free as an open source project. Manufacturers have to meet certain standards in order to get the official Google apps like GMail and the Google Play store, but those standards don't involve fees.
Google makes money directly off of Android via taking a cut of paid apps and running an advertising platform that developers can use for it. According to documents that have come out thanks to Oracle's lawsuit against Google, from 2009-11 Android made $543 million for the company that develops it. The Guardian estimates based on the rough numbers of activations Google has released that it comes out to a little over $10 per device. By comparison, Microsoft makes at least $5 per device on 70% over Androids out there thanks to patent royalties.
So is Android a profitable product to Google? It's still difficult to say even when you compare that $543 million over three years to the $38 billion of revenue the company brought in during 2011 alone (and consider that it's probably not the full amount of direct revenue anyway).
Google is not like most companies. It doesn't make most of its money by producing products and services and then selling them for more than they cost to make. It is perfectly content to pour money into products that it doesn't charge for so long as it can collect information about users and their habits and sometimes serve ads on them. It then uses that information to tune its advertising algorithms, the real core of the company. Serving up the most relevant ads possible to its users increases the likelihood of people clicking on them, which then maximizes revenue on the ads.
Android is a both an offensive and defensive play for the company. It's offensive in that Google can get information about users to use in its ad algorithms on top of the app and ad income. It's also a defensive play because Google wants to make sure there's a major mobile OS out there that won't shut out its services.
Google bought Android in 2005, well before the iPhone in a time when Windows Mobile was rapidly growing in the smartphone space. Google probably could envision a future where Windows Mobile dominated smartphones like regular Windows did on PCs, and the default search setting on there would be Microsoft's competing search engine. With mobile the future of computing, such a future would hurt Google's growth prospects drastically.
Windows Mobile obviously tanked and Windows Phone 7 is out there to replace it, but Apple is the big rival on mobile now. Apple does use Google Maps in iOS, but that may not last for too much longer. It also uses Google as the default search engine in Mobile Safari, but there's no guarantee that will last. It was even rumored heavily a couple years ago that Bing would replace Google as the search default.
Android may have indirectly provided the company more revenue than was reported in the court documents thanks to information from tracked Android users helping to hone the ad algorithms. It might also have kept the company from losing revenue. RIM's Blackberry would probably be No. 2 behind iOS absent Android and, for instance, the Blackberry Bold I have for work has Bing as the default search provider.
Google is probably content not to include such considerations when it comes to how much it might have to pay to Oracle in royalties for Java patents, but it does go into the consideration for the value proposition of Android to the company. Because Android provides some sense of security for the company's core business in the fast-changing and uncertain mobile computing market, its value to Google cannot be distilled down to a single number.