We can start with the idea that the $8 billion is trivial because of the trade deficit. As Politifact noted last month, a trade deficit doesn't mean that Mexico is sitting on $50 billion that we gave it. It means that we import more from Mexico than we export to Mexico -- all of us, not just the government.
According to the Census Bureau, the gap in 2015 was more like $58 billion.
That's billions in products we import from Mexico -- manufactured goods, fruits and vegetables, etc. -- that totals to about $60 billion more than what we send them. So the people holding that surplus on their end are the people who make those goods or grow those vegetables, not the government.
It's also worth noting that an $8 billion wall is an expensive wall, even to Donald Trump, who claims to be worth $10 billion but who may actually only be worth about $3 billion.
The gross domestic product of Mexico in 2014, for example, was about $1.3 trillion. That means that the $8 billion would be about 0.62 percent of the nation's GDP -- that Mexico would need to spend about $1 from every $162 it generated that year on the wall. The equivalent portion of the United States GDP from 2014 would be if we were asked to spend about $108 billion -- more than a sixth of the amount we spent on our military in 2015.
But Trump's $8 billion figure is almost certainly low. The Post's fact-checkers looked at that number on Thursday and found that there's almost no way that the costs could be held to that low a number. The concrete alone for a wall at the scale Trump describes would cost $2 billion, they figured.
The higher the cost of the wall, the bigger the bite out of the Mexican economy. It's important to note, too, that the country has been hammered by the drop in oil prices: Oil revenue makes up about a third of its budget. It's been operating at a deficit for some time, so the idea that our telling them they're paying for the wall will inspire them to add to that deficit seems unlikely.
So, there you go. Another look at an impractical statement from Donald Trump -- the latest in a very, very long line. Had New Hampshire voters had this information in-hand on Tuesday, who knows. Maybe enough of them would have been skeptical of his claims that he would only have won by 18 points instead of 19.