As it has been the hot topic on the amount of loss that Google has made on their investment on Motorola Mobility, there have been lot of calculations on how much really Google has lost on this purchase & sale of Motorola Mobility. For us layman the calculation is simple, Google bought Motorola Mobility at $12.5 billion & sold for $2.91 which means that a cool $9.6 billion loss. But according to the Newyork Times special report on this particular deal, the amount is completely different. I'll quote a small portion of that report, which has been republished by many technical blogs as well
And even $3.2 billion is not a loss considering the Google has got the chance to keep all the patents of Motorola Mobility. And if some one says, what about the millions of dollars loss of operation costs of Motorola Mobility in the last 2-3 years. Well, along with Motorola Mobility's patents, Google will also get the chance to keep the project 'Ara'(Modular cell phone) and also Lenovo will continously be paying royalty fee to Google for the use of Motorola Mobility's patents. All in all, the out come of all this Motorola Mobility's episode is that Google got Motorola Mobility's patents & Ara project for as cheap as $3.2 billion the same amount that Google paid for the purchase of Nest Labs(Home Products manufacturer).
Source: GSMArena & Newyork Times.