Also, Fox quotes a figure of $20 million in semi and full auto small arms being sold to Mexico in 2009. The Army pays about $1K for a M4 carbine. That would imply that the number of small arms we sold to Mexico is on the order of tens of thousands. That seems reasonable given that Mexico is trying to raise several new units. Of course, Mexico seems to arm most of its regular forces with G3s and other 7.62x51 NATO rifles, but they're likely buying some AR-15 platforms for police/paramilitary usage as well as all the other goodies we see on TV (explosives, grenade launchers, light machine guns, etc).
As has been discussed previously, there are about 30K arms recovered in Mexico every year. This may be a high number due to double counting. Only about 10% of those are traced back to the US, and of those, about 2/3 were "walked" in Gunwalker. That means that we only need to account to around 1000 weapons. It seems reasonable to think that around 5% of the direct foreign military sales are being diverted to cartel usage in a country where corruption is systemic.