Donald J. Trump vows to build a “great, great wall” across our southern border.

It’s going to cost as much as $20 billion — give or take a few billion bucks. How much money does the president have on hand to start the job?

Department of Homeland Security officials say they’ve got about $20 million on hand, in the bank, to start the job.

The gap between 20 billion and 20 million dollars is, um, really yuuuge, man!

Where’s the rest of it going to come from? Trump says Mexico will pay for it. The Mexican government says no … it won’t pay. Can the head of one sovereign government force the head of another one to do something he doesn’t want to do?

I guess we could go to war with ’em, right?

That won’t happen. Quite obviously.

According to Reuters: “Trump has said he will ask Congress to pay for what existing funds cannot cover and that Mexico will be pressured to pay back U.S. taxpayers at a later date.

“Republican House Speaker Paul Ryan has said he will include funding for a border wall in the budget for next fiscal year. He has estimated the cost to be between $12 billion and $15 billion.”

The Ryan estimate falls short of what Homeland Security officials have said; they place the cost at more than $21 billion.

This wall-building stuff is making my head spin.

Trump continues to court the support of fiscal conservatives. But he wants to spend $54 billion additional on defense spending, while cutting other programs to pay for boosting the Pentagon budget; he wants to spend $1 trillion on a road and bridge rebuilding program.

Oh, and he wants to cut taxes, too!

What does that do to the national debt? The annual budget deficit?

Does the president pile more debt on us while blowing the budget apart? Hey, I think he said the Obama administration’s “disastrous” fiscal policy was something he intended to fix.

Construction of this proposed wall, so help me, is going to cause many more headaches than it is intended to cure.