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.