I think it's unlikely but interesting.
Financially, while the economy was on the way up since '83 there's no way in hell people will be happy with such a massive purchase unless it can show almost immediate results in terms of profitability. I don't know how much oil can be exploited immediately in Baja or in the Sea of Cortez, but for $200 Billion it had better be a lot.
Secondly, the purchase would be just that for some time: a purchase...
Read: A territory a la Puerto Rico.
This both exacerbates and alleviates the immigration question in my opinion. On the one hand, Mexican immigrants might split between flocking to the U.S. both legally and otherwise as OTL, and flocking to U.S. Baja to get government oil rig jobs; a choice that the U.S. government might actually support.
I know some older folks who are moderately well off who got a place down there on the sea for fishing and tequila drinking until their ticker ticks its last. I don't know how long ago the American expats in Baja phenomenon began, but with Baja a U.S. territory I could see a drastic increase in American workers, tourists, and retirees from day one. This historically goes a long way towards eventual statehood. The idea that such a new territory could be made a state before Puerto Rico though would be absurd to Americans, Puerto Ricans, and Baja Californians so expect Puerto Rican statehood pressure to increase.
Would both territories become states by 2013, ttl? I can't say. I will say that in such a TL that it would be better and more dramatic from a writing perspective for Puerto Rico to receive statehood sometime in the nineties, and for Baja to receive the same by the present day.
As for the cartels and terrorist concerns, I'm sure as a territory the government is able to get away with sending police and military forces suitable to such an enterprise. (Though keep in mind I'm not sure how much of the cartels and terrorists exploiting the border story is non-fiction... Certainly the cartels are more likely realistic threat, but I don't have the facts on what their presence in most of Baja is. I'm aware of first hand accounts of them having an arm via gangs that operate along San Diego/Tijuana, but whether this would increase or decrease ttl is debatable.)
The more I write the better an idea I think it is; however it is cost prohibitive in the short term. If Mexico is hurting that badly the U.S. could get away with a better offer.
The effects on immigration reform are astronomical, though. Mexicans now have a place to go to "naturalize" before moving into California or Arizona. It's much easier for Puerto Ricans to immigrate legally than it is for Mexicans, as far as I know, and Mexicans who move into a recently Mexican U.S. territory would have an advantage over fence jumper types.
I think it's a cause for Reagan's successor to get hounded in the general, but ultimately another positive notch on the semi-fictitious but always fascinating belt of the Reagan Legacy.
EDIT: Maybe a slightly earlier POD would be required: one that makes Mexico a lot worse off and makes America a lot better off. I.e. A financial collapse in Mexico, and an earlier bounceback in the U.S. economy under Reagan, maybe?
