看曼昆的经济学原理,他讲的一个边际的例子没看懂.

看曼昆的经济学原理,他讲的一个边际的例子没看懂.
As another example,consider an airline deciding how much to charge passengers who fly standby.Suppose that flying a 200-seat plane across the country costs the airline $100,000.In this case,the average cost of each seat is $100,000/200,which is $500.One might be tempted to conclude that the airline should never sell a ticket for less than $500.In fact,however,the airline can raise its profits by opportunity cost whatever must be given up to obtain some item marginal changes small incremental adjustments to a plan of action thinking at the margin.Imagine that a plane is about to take off with ten empty seats,and a standby passenger is waiting at the gate willing to pay $300 for a seat.Should the airline sell it to him?Of course it should.If the plane has empty seats,the cost of adding one more passenger is minuscule.Although the average cost of flying a passenger is $500,the marginal cost is merely the cost of the bag of peanuts and can of soda that the extra passenger will consume.As long as the standby passenger pays more than the marginal cost,selling him a ticket is profitable.
我的问题是：Why the marginal cost is merely the cost of the bag of peanuts and can of soda that the extra passenger will consume?为什么说一个额外旅客消耗的边际成本只是增加了一瓶苏打水,一带糖炒栗子?难道它本身不需要有$500的实际成本么?