First Trump bellowed that Mexico will pay for the wall. At his rally’s he would say ” who’s going to pay for the wall!”, his minions would chant, Mexico! Mexico! Then after the crowd of rednecks were fed up with that chant, they would revert back to their favourite chant, “Lock her up, lock her up”.
What made Trump think that Mexico was going to pay for the wall is still a Great mystery. He had no legal basis for making such a claim.
Fast forward, now ‘The Donald’ is threatening to shut down the federal government if Congress doesn’t fund his wall. Estimates put the cost of the wall at anywhere between 10 and 20 billion dollars. Congress will not be enthusiastic about funding the Trump Wall.
In the warped mind of Trump this is the walls he envisioned:

A hundred feet high manned by remote anti-tank missiles and laser beams.

With Apache helicopter gunships patrolling the wall, prepared to open up on any rapist illegal Mexicans that venture near the wall.

Trump having another fantasy dream with another great wall while the Mexican president agrees to pay. He thinks of himself as such a great deal maker.

Trump having another wall wet dream.
Ultimately, Trump won’t get federal funding for the wall. He will have to pay for it with his own money. A downsize will happen because Trump doesn’t want to have to declare bankruptcy again. Even Trump can’t afford the “Great Wall”.
The downsized wall:

Or maybe something like this

At least it will keep out illegal Mexican Donkeys.