The farmer will realize that he can save this $100 cost by entering into a contract whereby he pays $50 (plus some extra enducement, say $51 total) to the railroad in exchange for the railroad installing the spark arrestor.

• 农民将会意识到，他可以支付50美元（加上某些额外的费用，假定共计51美元）给铁路，作为交换，铁路将安装50美元的阻火器。