Donald Trump's desire to "build a wall" is incoherent.
However, it's a perfectly coherent idea: it has a concrete meaning that is easy to understand.
What do you think is incoherent? A plan to build some kind of physical wall (barrier), or a serious plan to keep people from crossing the border outside of the normal immigration process, or both?
Whether it is realistic, plausable, beneficial, cost effective, or just plain stupid is another question.
On the other hand, given the trade deficit he could say, we're pulling out of Nafta, unless you kerb your unauthorized border crossings.
Alternatively he could fine the unauthorized border crossers they catch and partially fund it that way. Or he could institute temporary permits (seasonal whatever) and impose a fee for those. Also fine employers who hire undocumented workers and fund it partially that way.
It really depends on what he means by "Mexico" and "pay". But if he's flexible, and he appears to be, it could be possible.
Trump doesn't help his case by simply headlining "and Mexico will pay for it" over and over, but he does mention practical suggestions (even if you remain opposed to the fundamental concept) from time to time on most of his policies.