What you are proposing, basically becoming the infrastructure, is something that has been tried before, by Russia. It didn't work. Why? Because all they did was set the market price and the growers let the Black Market drive it up from there. It is black label economics, but still economics.
There is no way to enforce the growers to sell to the U.S. unless, honestly, unless it was at gunpoint; and our current political regime simply wouldn't let that happen.
I like the idea, and yes, it would be cheaper, but no one ever said that acting in the country's best interest and doing the right thing was ever the same, or cheap.