Trump's Wall Cost: Is the Trump administration underestimating the price for a new Mexico border? - Build the wall!" became perhaps the best-known chant from the campaign of President Donald Trump. Now, he's trying to make that U.S.-Mexico border wall happen (much to the chagrin of the entire state of Texas). Trump claimed the wall would cost roughly $12 billion, while Senate Majority Leader Mitch McConnell suggested in January a price tag of up to $15 billion. By February, the Department of Homeland Security had its own estimate. Unsurprisingly, the cost had jumped higher once again. The new tag: $21.6 billion. But the number just seems to keep climbing.