Comparing John Deere tractors and Google's car is nonsensical in my opinion, but that doesn't change the fact that this technology can seriously improve farming.
I agree with you - it's an odd comparison. That $20k figure they list isn't the systems cost - it's the guidance cost. A modern farming tractor is a six figure item, often pricing out between a quarter and a half million dollars. They also rarely go over 10mph. They also never have to worry about traffic. So yeah - autopilot has been around a long time, too. It's sort of a different problem when you're at 35,000 feet and need to cover two thousand nautical miles in a straight line than when you need to worry about running over pedestrians in the Castro. Tractors went gonzo because farming has contracted. A family farm in the '20s was 5 acres; a family farm these days is 5000 and it's heavily reliant on petroleum inputs and automation.
The biggest problem I had with comparing a tractor and Google's car is that the latter is on an entirely different level. These vehicles have been running for some years now in Rotterdam: It's a fully automated shuttle service between a business park and a train station two miles away. Started operating in 2005. Just like a farm, the vehicle operates in a closed-off environment. The shuttle has its own roads, which means that it doesn't need to predict human behaviour. It only needs rudimentary object recognition. Whether it's a tractor or a shuttle, driving a vehicle in such a simple environment is baby steps compared to what 'real' automated driving is about.