You don't understand what I meant by "incoherent". I am very pro free movement. I consider my homeland to be the multi-ethnic region consisting of the SW USA and northern Mexico and viscerally hate the idea of some man made wall dividing it.
However, it's a perfectly coherent idea: it has a concrete meaning that is easy to understand.
The desire to "build a wall" is a reaction of locals along the border to the realities there, and that reaction diffuses through the rest of the population who aren't on the border but who see problems that they perceive (or are persuaded to perceive) might be solved by curtailing illegal immigration. They may be wrong about, or miscalculating, the ultimate consequences of a wall, but the perceived need for some kind of "wall" is coherent in an immediate sense.
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?
If he means it's going to come out of their pockets directly, he's deluded.
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.
In fact Trump has mentioned ideas such as a special tax on any money sent to Mexico, which is certainly plausible.
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.
I think you are getting brow-beaten by a kind of pedantry here, sorry for that. I'm taking you to mean that his statements are those of a buffoon that no one in intellectual society should take seriously. I agree with you.
He is coherent in the fantasy world of his supporters. And he has millions of them. And their fantasy is a possible reality soon. I laughed at Trump when he started to run for president. It is not so funny anymore, his odds have gone up.
However, it's a perfectly coherent idea: it has a concrete meaning that is easy to understand.