If we can't prevent farmes to grow poppies when it's "illegal" to do so, what's to prevent farmers from taking money to "not grow" and still grow opium?

I suppose, if one buys into the idea that this is mostly the result of a free marked (as opposed to an intricate web of political, economic and organized violent crime) -- one could offer to pay opium prices for food crops. But then you'd probably have to destroy that food, in order to prevent people from selling the food back, and collecting double payments on "not growing poppies"...