Hack attack: Will driverless cars be safe from cyber-attacks?

Hacking involves a lot of research. The first step is finding as much public documentation as possible, then getting access to a vehicle, and finally spending as much time poking at the vehicle's interfaces as possible. However, we have to look at this from a return on investment (ROI) standpoint too. If my costs to buy or acquire the vehicle and spend a week or two with it were high, then I might not have reason to simply use this exploit for fun, but may have motivation to hold onto it. But if I wait too long a fix for the exploit may be available, thus negating my time and effort.