It starts just south of Beaumont, Texas, and runs almost 2,000 miles to just south of San Diego. The hard part would be fencing the Rio Grande, which is almost 1,200 miles of the border demarcation.
A wall can be built. The Ming Great Wall, which is not the entire Great Wall of China but was the most well-built, was 5,500 miles long. Based on several sources, it took 200 years to build and may have involved 800,000 people. Building a wall between the United States and Mexico might be done in a few years and with many fewer people. The undertaking would be expensive, but no one knows how much. Some portion of the border is already walled. But much of it is not planted deeply enough to prevent tunneling.
Building the Second Avenue subway in New York City will cost $4.5 billion, and it will only be a few miles long. The project is late and has gone on for several years. A lot of rock, apparently, and a lot of expensive machines.
The California High-Speed Rail Authority plans to build a route about 600 miles long. The total cost will be $91 billion, and it will take until 2033. That is, if the state can get all the money.
The cost to build the Tappan Zee Bridge across the Hudson River for a length of fewer than five miles will cost $6.4 billion, and another $2.9 billion for bus lanes.