it's not the cost that's the problem.
fwiw, i would support deficit 
spending for infrastructure projects. if they're going to build it, i'd 
prefer they build it with public funds than private funds.
so, not only is that not why 
they're not building it, but it's also not a reason i'd oppose it. it's 
actually a reason i'd support it; if you ignore the context of the 
scenario, it would actually be good for the economy.
the reason they're not building
 it is that it would cut off the flow of cheap migrant labour, which 
would be disastrous for the surrounding agricultural industries.
the banks that control donald trump don't want a wall. they want a 
porous border that is easy to get across - for the people they want to 
allow across.
there are just a lot of very stupid people out there that fell for a lot of very obviously contrived nonsense.