White rice is just brown rice with the bran removed. The bran has oil in it which can be extracted to form another product, rice bran oil. By splitting the cost of production between two products a lower price can be provided for the white rice.
Another factor is that the oil in the bran means that brown rice can spoil more easily than white rice. Shorter shelf life and more costly transportation means brown rice pushes brown rice to be more expensive.
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