President Trump also lays out another potential path in his Executive Order:
"The head of each executive department and agency shall identify and quantify all sources of direct and indirect Federal aid or assistance to the Government of Mexico on an annual basis over the past five years, including all bilateral and multilateral development aid, economic assistance, humanitarian aid, and military aid.”
It goes on to submit that list to the secretary of state within 30 days. The order then gives the State Department another 60 days to “submit to the President a consolidated report reflecting the levels of such aid and assistance that has been provided annually, over each of the past five years.”
The Mexican government and military received nearly $420 million in assistance in 2013, with most of the money going to economic development.  If President Trump is in power for 8 years, eliminating $420M p.a. is approx. $3.5B which is a good down payment on the wall. Art of the deal, anyone?