Wondering about a PV panel mounted on 4 wheels dragging a hoe between crop rows. Simple computer vision on a Pi Zero should be able to work out where the crop is and steer the machine. The cost should be below $500 so if it can do an acre a week and last 5 years it would work out cheaper than herbicides. With improved vision and a laser surface insects could be zapped.
You have to remove the “simple”. At the moment, nothing about vision is easy, and vision + navigation in real world situations is still in research labs.
There is a machine that tries to do what you say, for that price, from the inventor of the Roomba. It just cuts everything above a certain height using the assumption that the wanted plants are bigger. It can’t see the difference between plant or weed.