According to the SEC filing documents released by Motorola Mobility yesterday, Google actually outbid itself by 33% when it purchased Motorola Mobility. Apparently, at first Google made an initial offer of $30 dollars per share for Motorola Mobility on August 1st. Then, on August 9th, Motorola requested a new bid target of $43.50, and Google raised their bid to $37 dollars per share. However, here's where they ended up tricking themselves into a larger bid. Later that same day, Google was worried that Motorola might be snapped up in a public auction, even though Motorola wasn't accepting bids directly from other firms, so just to be safe, Google upped their bid to $40 dollars per share. This resulted in them inadvertently out-bidding themselves by 33 percent, an increase of $3 billion dollars, resulting in the final price of $12.5 billion.

Interestingly, the Wall Street Journal was able to shed some more light on the subject that delves even deeper into the real reason this deal was so important to both Google and Motorola. Here's a quote from the Engadget article with the details,
The documents also reveal that patent-related issues were at the forefront of discussions from the very beginning, when Google's Senior Vice President Andy Rubin met with Motorola Mobility CEO Sanjay Jha to talk about their mutual concerns, way back in July. According to the Wall Street Journal, these talks eventually convinced Jha that his company would be better off under Google's stewardship, amid fears that Moto could get swallowed by the stormy seas of patent litigation -- anxieties that the exec made all too apparent just four days before the merger was announced.
It appears that both Google and Motorola were concerned that Motorola would become so weakened by all the legal battles, that they might be ruined or gobbled up by other companies. This was obviously a big deal to Motorola, but it was also important to Google, because Motorola helped pioneer the Android ecosystem. It would have been a dangerous thing for Android as a whole to let the legal battles put Motorola in the position of a hostile takeover. So, the ultimate solution was for Google to swallow up Motorola Mobility before someone else did, allowing Google to continue to support the Android ecosystem.

Source: Engadget and Wall Street Journal and Bloomberg