Fair enough, but wouldn’t soldiers be a cheaper? Maybe we can have the CBO run the numbers :-)
Soldiers used in civilian roles is a possible violation of Posse Comitatus. Something like that may be done, but if A VERY LARGE WALL is constructed, the drug thugs will be forced to smuggle drugs and drugs will become much less profitable.
If you take the unbelievable profit margins that the cheaply built border fence creates and turn it into a very-difficult-to-penetrate wall of steel, drug running suddenly becomes very expensive. You don’t only need a 4X4 to run drugs anymore.