It's different for different products. The reason for not making that manhole cover is becuase, that would take manpower to do it (I don't know how many men, but for the sake of argument, let's say 3). It is more effeicent for those 3 men to buy a manhole cover from Mexico and to spend their time doing something else, than for them to make it themselves.

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"The wisest men follow their own direction." - Euripides