Okay...I gotta admit that when I first checked this thread again, it was up to five pages and there was a heavy debate* going on already. Rather than mingle and attempt to reply everyone individually, I gotta say I thank everyone for voicing their concerns and opinions on the matter, even though I don't always agree (which...would've been impossible in this sort of situation).
As it stands, the dust is more or less settled on this topic (alas: it's more because Trump is busy creating NEW controversies rather than solving other ones). Let's recap things a bit:
-both American and non-American tempers believe that it's US citizens who will end up paying for it in the end, though the former has a bit higher chance of believing it. Note that this small poll is far from done on a large enough scale to get proper statistical data (gbatempers don't represent enough of a diversification on populae in the first place), so please don't get too overeager to tell this to the world.
-the "most immigrants come by plane" is an important argument, and a heavily debated one. As it becomes clear with Donald's (even more) controversial act of prohibiting people from Muslim countries**. I have no doubt that as this wall thing goes through, even simple vacations to the US will become extremely hard for Mexicans.
-perhaps more interesting to me personally is what is NOT mentioned: the loss of jobs due to automatisations and off-shoring. I just read a book about this***, and asides predicting that as a species, we're on the verge of huge, systematic unemployment because of automatisation, he also makes a very valid observation on the subject of immigration. He basically wonders why everyone seems to hate Mexicans who come over and do jobs that US citizens won't like to do (cleaning, janitor services, fastfood operator, and so on), but nobody makes a fuss when countries like India "steal" all sorts of jobs that US citizens would more likely do (telemarketeer, tech support, insurance, these sorts of things). In other words: what's the point of closing physical borders when the virtual ones remain to be wide open?
-unfortunately, I predicted the outcome in that "we'll pressure them into doing it" option rather correctly: Trump wants to build his wall by using an import tax on Mexican goods. The problem with this, however, is that this would be a violation of a treaty that he conveniently placed in a bad light prior to proposing this. In other words: NAFTA. The funny thing is that I've read about this treaty about a couple months before the elections. In that book****, the (American) author feels almost apologetic towards Mexico, as he thoroughly describes how Mexico couldn't compete with the subsidized food that America dumped into Mexico (thus effectively pushing poor farmers off the market) while abusing local antidump-laws to fend off the import of cheaper Mexican tomatoes. Granted, the book is ten years old by now, but when asked to believe a nobel prize winner or a business man who lies more than he speaks the truth, I chose the former. In other words: NAFTA never was a one-sided deal in favor of Mexico. And with that being a lie, I'm not so sure what will happen if this income tax (and with that: the total annulation of NAFTA) goes through. But the chances of it ever being sufficient of completely building that wall are low. So low I honestly wouldn't trust it as an American taxpayer.
-though I'm thoroughly against that wall, I'll give it my opponents that there is a good side to this situation: employment. You can't build a Chinese wall in your backyard without a huge amount of workers. No matter how much or little use that wall will end up having, employees to build, maintain and/or guard the wall gives them something to do. And cynical as I might be about the purpose, giving people jobs helps the economy.
*which differs from a discussion in that everyone already had their opinions formed and wasn't going to budge.
**according to him, the fact that they're Muslim countries has nothing to do with it. According to everyone else, it has everything to do with it.
***Rise of the machines, by Thomas Rid
****making globalisation work, by Joseph Stiglitz