To be fair, I think you should also include cartel spending and manpower in the Mexico figures. Things could get messy sure, but if we're not fighting the cartels, the war would be a foreign policy fuckup on the level of Afghanistan before it even started; which of course I would not put past us.
The thing I've always said when people bring up war with Mexico is look at Russia and Ukraine. Russia gets stuck into a big war with their neighbor and their ability to project power suffers and their allies (Iran, Armenia) start having to eat shit. Maybe it'll be worth it for them, but if you don't anticipate any downsides you're not being realistic. And sure, it's a different situation, Ukraine has the second biggest military in Europe after Russia and it's a protracted land war. Mexico, on the other hand, is a nearly failed state shot through with corruption and organized crime, and an invasion has the potential for a very long tail insurgency...hmm, does that sound familiar? I agree that defeating the Mexican army and government conventionally wouldn't be so hard, but it doesn't make sense that a weaker Mexican state should be our goal. Also, look at how we and Europe have taken the opportunity to fuck with Russia through the war. China and others will absolutely take that same opportunity. I know, the cartels are already our problem, I know, China is already fucking with us. Doesn't mean you have to get baited into Afghanistan 2: This Time We Share A Land Border (available in IMAX in select theaters).