Infinite! Well, almost. Imagine, people spend on average 10 hours a week in their car. They don't have to drive the googlecar, so it can be fitted with screens instead of windshields, and you can push onto them 10 hours a week of endoctrinement^W advertisement (for "free" driving)! Priceless!
IMHO - Google's role is to get people to spend more time online. They make their money from search...so, the more people online, the more money they make. Phones, glasses and cars are all designed around this objective. If you aren't driving a car, then that is more time that you are spending the net and thus, more money that they can make from you.
Assume 60 million cars produced each year, 10% with Google autonomous driving software and sensors, with a $1,000 per car licensing fee. That's a $6 billion per year business with very high margins.
If net margins are 50% and this division commands the same multiple as Google itself, then the division would be worth about $100 billion. That's about 25% of Google's current market value.
Today, there's talk of Uber being a $200 billion company[0]. Google Ventures is an investor in Uber, and if they could combine the autonomous vehicle tech with the Uber network, what would that be worth?
[0] http://www.businessweek.com/videos/2014-06-13/uber-could-be-...