If you don't want to outright cheat but still want easy money you can abuse the buy/sell land mechanics. The price for land is the same for both buying and selling and doesn't factor in the state of the crops. Let the AI plant & fertilize and buy the plot when ready to harvest, even if you have to take out loans. Harvest/sell crops then sell the land back for the same price as you bought it. Way more money that way than contracting to harvest for the AI owner. A lot of times the AI is lazy and won't plow or lime their fields so the yields might be lower but still it's easy money, especially if your combine is sitting idle waiting on your own fields.