You must not have seen some of the border patrol documentaries that are out there. It's a different world and a wall is not such a "stupid" thing. Surely you don't need to be told Trump isn't suggesting the U.S. send Mexico the bill and trust they will send us a check.
Mexico could find itself paying for the wall a number of ways. 20% tax on Mexican imports would generate over $100 billion in 10 years. An estimated $25 billion a year in laundered money is wired from the U.S. to Mexico. Since the U.S. seems agreeable to allowing people to live here illegally, there is a thought that at least we should tax their ill-gotten gains. A border adjustment tax would generate $12 billion a year and then there are travel visa fees that could be increased, etc.
Some voter's believe everything he says word for word and take it literally. Especially, his legion of followers.
A lot of families that live along the boarder have had land there for several generations. For the US government to come in and take their land to put up a useless wall is disrespectful and encroaching. Many people who live along the boarder don't want a wall.
The US consumer pays taxes on imports, not the manufacturer. From a micro economic prospective, higher boarder taxes would hurt a lot of boarder towns who depend on people everyday to cross the boarder to purchase goods, services, and work. There are a lot of different travel visas so that could have a big impact on our nation as a whole. Technology, health care, and educational companies would be affected if the number of visas given were limited or fees raised.
If marijuana was legal and we ended or lessened the wellfare state, this wouldn't be a discussion. If we pulled our troops out of occupying other nations and trained/stationed them along our boarder, that would put a stop to a lot of this. It would save our country lives, money, and stature in the world.