Its not a matter of money. Trump agreed to 1.3 billion and was ready to sign off on Congressional legislation until the likes of Coulter and Rush belittled him into changing his mind. The Democrats are more than willing to give him what he originally agreed upon.
Why is it impossible for them to come up with $5 billion, which amounts to .12% of the Federal Budget, or $29 billion, which amounts to .69% of the federal budget to complete the project? It is a trivial amount to fight over. I think Darth hit the nail on the head. The thought of the wall working scares the shit out of them after all their false claims and rhetoric.