The US president had initially insisted that the spending bill, which must be passed by the end of April 28 to avoid a government shutdown, include funding for the wall. He appears to have somewhat backed down from that position. However, he has in no unequivocal terms said he will build the barrier, whether he gets the funding now or later.

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How much it will cost is very much an unsettled matter. Here’s the history of how the wall’s price tag has fluctuated among experts, politicians and Trump himself.

September 2015—Then candidate Trump said that while some estimates put the wall’s cost at $10 billion, he could build it for $4 billion.
November 2015–In less than two months time, his estimate went up to $6 billion to $7 billion. (The cost for the wall if someone else were to build it doubled to $20 billion, per Trump.)
February 2016
At the beginning of the month, Trump added another billion to his quote to make it $8 billion.
Just a few days later, he upped the estimate to $10 billion to $12 billion.
March 2016—Trump reconsidered, again, putting the cost at $5 billion to $10 billion.
July 2016—By this time, others were jumping into the cost-estimation game. In a widely quoted analyst note, Bernstein Research put it at $15 billion to $25 billion (pdf.)
January 2017—Senate majority leader Mitch McConnell said it would cost between $12 billion to $15 billion.
February 2017—A leaked report from the Department of Homeland Security put it much higher, at $21.6 billion.
April 2017
On April 18, a group of Democratic staffers at the Senate Committee on Homeland Security and Governmental Affairs published an estimate of $66.9 billion. They came to it by extrapolating per-mile costs based on Trump’s initial request for wall funding, which covers less than 200 miles.
On April 21, Trump told the AP that he could build the wall for $10 billion or less.

Even the highest among those estimates don’t come near what Late Show host Stephen Colbert calculated with the help of some experts, including an engineer, an architect, and a concrete-business owner. They estimated it would take $2 trillion to meet Trump’s superlative specs. (The wallpaper suggested by the interior designer to make the wall beautiful was not accounted for.)

It was an exercise meant for laughs, but it hints at the double folly of spending billions to build a 2,000-mile barrier and expecting it will keep everyone and everything out.