STRATFOR has published an article entitled Mexico: Economics and the Arms Trade. They made a very good point in saying that with semi-automatic rifles and ammunition prices in the United States so high, the large Mexican gangs have little incentive to shop over the boarder when importing from overseas is so much cheaper (even ignoring the fact that US civilian weapons are not fully-automatic, unlike overseas imports).
In fact, due to this surge in demand, it is downright difficult to locate many types of assault rifles and certain calibers of ammunition, though a lucky buyer might be able to find a basic stripped-down AR-15 for $850 to $1,100, or a semiautomatic AK-47 for $650 to $850. Of course, such a gun purchased in the United States and smuggled into Mexico will be sold to the cartels at a hefty premium above the purchase price.
By way of comparison, in places where weapons are abundant, such as Yemen, a surplus fully automatic assault rifle can be purchased for under $100 on the white arms market and for about the same price on the black arms market. This difference in price provides a powerful economic incentive to buy low elsewhere and sell high in Mexico, as does the inability to get certain classes of weapons such as RPGs and fragmentation grenades in the United States. Indeed, we have seen reports of international arms merchants from places like Israel and Belgium selling weapons to the cartels and bringing that ordnance into Mexico through routes other than over the U.S. border. Additionally, in South America, a number of arms smugglers, including Hezbollah and Russian organized-crime groups, have made a considerable amount of money supplying arms to groups in the region like the FARC.
I previously wrote that a significant proportion of the US-manufactured weapons captured in Mexico were in fact civilian hunting weapons, not semi-automatic, and legal to own and purchase in Mexico.
Thanks to Michael for sending me the article.