Well, it's damned late and I'm exhausted - so source/confirm things yourself, but in short, and potentially badly:

With Mexico, it's said that 22 billion dollars are crossing the border just from immigrants and illegal aliens sending it back home. And I think our overall trade with them is something like ~300 billion...much of it from us buying their exports. Plus - I think there's also foreign aid involved on top of all that.

Suffice to say the business they do with America is a significant portion of their GDP.

So - yeah, it's plausible given our much stronger economic position we could either cajole them into financing this stuff or increase fees related to immigration, visas, and stricter enforcement about international finance (people sending MoneyGrams to Tijuana or whatever) to "find" that money.

And additionally, to a less quantifiable extent, just being able to significantly slow illegal immigration and all its associated costs will save us a damned big amount of cash, too...so over time, the savings pays for said mega-wall as well.

Ethically, morally, etc? I don't think I could head over to Canada and expect particularly generous treatment from their authorities.

China, it's a similar economic situation. They don't import our products to anywhere near the same degree we import theirs, and when they do import American made stuff, it's loaded with tariffs, taxes, and regulatory red tape. It's one hell of a trade deficit we run with them.

They also keep their currency devalued wherever possible to exacerbate this situation - if $1 USD spends the same in both countries, there's not much incentive to import container loads of Chinese made product.

So American businesses not only get undercut by a country that has no objections to environmental conditions necessitating oxygen tanks, salaries and hours for their workforce that are ludicrous by any sane standard, and immense regulations and taxation on the domestic side here - but they can't even sell their goddamn stuff in China due to tariffs that they levy.

On top of that, they either ship American products over and copy them, or just copy them there (see also: Ghost Factories/Shifts) and sell them on the world market with little reprisal.

They need our business - I don't think they want to experience what happens when they've gotta shut down factories and cut production. And Trump is right - we have terrible fucking negotiators when it comes to trade deals with them - industry folk say the same.

Far as Muslims, it's a touchy subject. Letting in refugees with the level of oversight we're capable of currently - almost none - is a pretty big risk.

Letting in Middle Eastern folk from various countries on visas carries a similar, if smaller risk - hell, ISIS has been known to overrun towns and get their damned passport printers and blank books. Guy in Thailand just got nailed for producing fake documentation for migrants.

Until we can address these risks effectively, and while the Middle East is so utterly unstable (it's not like you can just call up the local governments in some of these embattled areas and ask about Ahmad) - it's not a bad idea to say "sorry, but no" for a while - we're seeing more and more attacks here that almost never happened before; at least in the past we had domestic nutjobs and extremists.

And take this as you will - but I've had a lot of experience with the domestic Middle Eastern population - their kids all hate each other. Lebanese hate the Iraqis, the Iraqis hate the Yemeni, the Iraqis and the Yemeni hate the Jordanians, most of them all hate us Judeo-Christian white folk...it's a clusterfuck.

And that's supposedly "Americanized" second/third-generation.

Many don't integrate to any substantial degree, they don't hire or fraternize outside of their ethnicity or religion, racism internally and externally is rampant, it...well, it goes on and on. Mind you, not always the case, but frequently.

Point is, if it's that much of a gorram mess for people that have immigrated and by all rights should have integrated, it's a powder keg situation importing sectarian people from warzones.

So - consider that it's just plain dangerous and ask yourself if you'd want to get shot, blown up, or the like to test the hypothesis that nothing will happen.

Anyway, hope that helps a little.