You might check your math here buddy. Sure a combine (which is called that because it does three jobs all at once, reaping, threshing and winnowing) may cost 100$/hr over it's useful lifespan, but it replaces much more than one person (more like 150-300 people), especially when you consider that harvesting is extremely time-sensitive, sometimes in a matter of hours...

Edit: after some research and calculations, my estimate is that a single modern combined harvester can replace between 350 and 700 men.