Who can use the food.
But don't really need it, and can shop elsewhere easily to get good prices.
And I'm gambling that they won't buy food at 50 gold per 100.
Edit: As a matter of fact, the Lurias as a whole, once they reach full production in their regions, would likely produce a surplus of up to three times D'Hara's deficit if they fix their monster issues, or otherwise at least 1.5 times. Just goes to show that if the Lurians want some spare bushels now to be on the safe side in case of bad harvests and unmanageable hordes, they will continue being net producers.
Edit2: In fact, I calculate that the Lurias' surplus (both of them combined) will be greater than Madina's.