Rent on farmland isn't exactly a quick route to riches. $100/acre is probably very generous, and even assuming all 40 acres are arable also is probably overly so as well.
So, rent is likely 4 grand at best. From which you pay taxes, upkeep on any buildings, insurance, mowing for any labs that isn't farmed or leased, etc etc. So...not much in profit at the end of the day.