An armed Predator sale might be a tough thing to pass through your foreign military approval process. Once the MQ-1 arrives in Mexico, who really controls it?
Well the things is, Mexico's skies have a fair amount of UAS platforms over their airspace. I'm not sure how the legality of all it works, or if it's even openly admitted, but DEA, DHS, and I believe DoD all has UAS platforms keeping an eye on certain areas, and I'm sure some of that information gets shared with the Government in Mexico.
I'm also aware that other Federal agencies, like the State Department also have done flights over Mexico, but from what I understand, in manned aircraft as simple as Cessna's or Ranger helicopters. I've even heard reports of some aircraft taking fire and that some cartels are rumored to posses MANPADS even, though much of this information you WILL NOT find on the Internet.
Besides, why spend money on drones, when your northern neighbor (U.S.) is essentially the drone capital of the world, and they are willing to share their ISR, or at least some of it? I'm sure they have no qualms about us picking up the bill.