Manchester, New Hampshire (CNN) Donald Trump estimated on Tuesday that it will cost about $8 billion to build the wall he has promised to erect on the U.S.'s southern border if elected president.

Trump has vowed to force Mexico to pay for the border wall and said Tuesday on MSNBC that the hefty $8 billion price tag "is a tiny fraction of the money that we lose with Mexico" on trade.

"The wall is probably $8 billion, which is a tiny fraction of the money that we lose with Mexico. We lose a tremendous amount of trade deficits. We have a trade deficit with Mexico that is astronomical, much bigger than that. We will get -- and I say it also is also part of my plan -- Mexico is going to pay for the wall," he said.

Trump's proposal to build a wall on the U.S.'s southern border and his hardline immigration policies have become central to the billionaire's presidential campaign. But Tuesday marked the first time he has tacked a specific dollar amount to his border wall proposal.

Trump said he came up with the figure by "multiplying the number of miles by a certain number."

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