Looking at the only-slightly-wider chasm between the ag. commodity price of wheat, and the retail price of wheat grains - note that wheat needs less than 1/2 of the steps you list - I'm thinking that the actual processing only justifies a tiny fraction of the markup for rice.
Since "Sorting" means removal of pieces of stem and such and milling removes the hulls, the price per pound is sure to increase more that a tiny fraction unless the pieces of stem or the hulls are valuable, which I doubt.
Also, "drying" sounds like it could easily halve the weight of a grain of rice. Unlike wheat, rice spends its life in a very wet environment.
