but I'm trying to find the cost savings here . . .
The cost savings is in the repeatable accuracy with minimal effort on the part of the farmer. Driving in exactly straight rows for hours at a time isn't easy and it's taxing on one's mental state. My father used to run a corporate farm in Colorado, and man he got grumpy because in haying season he was about the only one who run the harvester, no one else could drive it well enough and fast enough to get the job done, but he ended up working 20 hour days for about a week. If he had AutoTrac back then, Almost any one could have run it. If you factor in the time saved by avoiding his mistakes caused by sleep deprivation, and the collateral damage itself, and it would indeed save money. But there is also the added bonus of very little overlap. Imagine paying $X per gallon of fertilizer, but wasting 5% on every pass for dependable coverage. With AutoTrac you can drop that to 1%, and on a big operation with expensive chemicals those savings can also stack up quickly.
Also, just compare it to cruise control, I mean why do we need that if the driver still has to be in the seat? It is a creature comfort.
There are a bunch of things in the works to essentially make the farmer a force multiplier, but if I told you I'd have to kill you