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A wall would cost far more than $5 billion, so why have this fight?

President Donald Trump talks with reporters after reviewing border wall prototypes, Tuesday, March 13, 2018, in San Diego.
President Donald Trump talks with reporters after reviewing border wall prototypes, Tuesday, March 13, 2018, in San Diego.

As the government gets set to shut down in about 12 hours, there's apparently only one number that matters: $5 billion. That's the only line on Donald Trump's ransom note to Congress. If lawmakers agree to turn over the $5 billion for his border-wall project, he'll keep the government open. If they don't, he'll shut the government down -- "for a very long time."

But it's worth pausing to ask an awkward question: how exactly did we arrive at this specific figure?

In February 2017, Reuters published an interesting report on the project's price tag.

President Donald Trump's "wall" along the U.S.-Mexico border would be a series of fences and walls that would cost as much as $21.6 billion, and take more than three years to construct, based on a U.S. Department of Homeland Security internal report seen by Reuters on Thursday.The report's estimated price-tag is much higher than a $12-billion figure cited by Trump in his campaign and estimates as high as $15 billion from Republican House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell.

Trump himself gave the project a $20 billion price tag, while a Politico  report, citing estimates from Capitol Hill sources, added that some lawmakers believe the price "could be as high as $50 billion when all is said and done."

As Vox's Matt Yglesias noted yesterday, the White House put together a budget request last year seeking $25 billion for the project.

So, putting aside related angles -- I seem to recall Trump promising Mexico would pony up money for this -- why is the fight over $5 billion?

As best as I can tell, Trump picked an arbitrary number that sounded good, knowing that it'd be tougher to get even House Republicans to go along with the $25 billion request the White House made a year ago.

Sure, $5 billion won't build much of a wall, but during the president's re-election bid, he'd be able to tell his followers, "Against all odds, I secured the funds needed to start the wall. Give me a second term, and I'll finish the job."

It won't be a great argument -- there's still the whole Mexican-financing thing to consider -- but Trump also knows that if he gets nothing, which now appears very likely, he's going to have a tough time explaining to voters why he failed so spectacularly on his signature issue.

With this in mind, assuming there is a shutdown, I expect Trump to start lowering his target price, in the hopes that Democrats give him something to help him save face. I have a hunch this won't work, but time will tell.