And what would be the benefit? Financially I mean. What's the return on investment? And how does it apply to the thousands of square miles of farmland?

There are precision farming systems that (according to their marketing materials) will allow you to precisely spray fertiliser only at plants, and weedkiller only at weeds. This saves money on chemicals, and helps the environment through reduced run-off.

Of course, adoption isn't universal [1] - a large farm will see a much faster payback time for the same bit of equipment, for example. And many farms will have other priorities for their limited capital; tractor auto-steer is much more popular than precision weed control.

[1] https://core.ac.uk/download/pdf/147418633.pdf

Nothing that goes into the field or on a tractor is going to be that cheap if you expect it to last more than a few days.