At a time when federal deficits are again expected to grow, $21.6 billion is not pocket change.

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No wonder President Trump wants Mexico to pay for it.

The wall along the southern U.S. border — really a set of fences and walls — would take 3 ½ years to build and would cost $21.6 billion, according to an internal report by the Department of Homeland Security (DHS) that was seen by Reuters.

That is much more than the $12 billion estimated by Trump during the campaign, and the upper bound of $15 billion cited by Republican leaders in Congress.

At a time federal deficits are again expected to grow, the amount is not pocket change. As the population ages and draws on Social Security, Medicare and Medicaid, the Congressional Budget Office is expecting the government to heap nearly $10 trillion onto the federal debt over the coming decade — and that is with statutory caps on domestic and military spending. Trump has vowed to bust through those caps on the military side of the ledger, something that would take an act of Congress.

Trump, in two tweets Saturday, promised that he would bargain the cost of the wall “way down,” according to a Washington Post report. Trump offered no examples of how he would accomplish that.

A DHS spokesman told The Post that the agency was still determining estimates and so any document about the cost was “predecisional.”