I have had recent experience on the agricultural use of GPS on my cousin's farm. Far from the simple crop seeding that we city folk think of in farming are the realities of pest and weed control. In broad acre farming, machines like sprayers often cover widths of 100ft (30m). This makes it difficult to align accurate coverage from the driver's position. Overlapping of spread wastes chemicals and the double dosing can raise health/quality issues of the crop when selling the produce. "Underlapping" (missing areas) leaves weeds flourishing, affecting harvest or pests remain alive to reinfest the crop.
The GPS is used to control machine alignment (eg exactly 30m from previous pass) and control the application rate of seed/fertiliser/chemicals by accurately determining ground speed.
The devices are quite specialised and only a few thousand (hundred?) of each model might be sold in a country like Australia. Consequently, the development and support costs can be high. From memory, my cousin paid around $A10,000 for his unit, which he considered to be good value for money. Accuracy was to within 200mm (8") which he was able to demonstrate by doing a return pass: the tractor wheel tracks aligned almost perfectly.