I wouldn't be so sure that Trump can't commence building the wall anyway, under emergency powers using the military under the orders of the Secretary of Defense. The legal challenges might not be able to stop that. The Secretary of Defense has an existing ~$700 billion budget. The wall would be peanuts.
Allows: Construction of roads and fences and installation of lighting to block drug smuggling corridors across international boundaries of the United States.
Hasn't the Whitehouse been claiming lately (against all common sense and evidence) that "The Wall" would have stopped El Chapo?