Regardless of the revenue source, congress must fund it. The president can't just look under a rock and say, Look, I found 35 billion dollars. I'm going to building a wall."
Revenue for any project is always an issue but the overriding issue is do we want it, not what it cost. 70% of the American public say the wall is not a priority and over half say it should not build. There is opposition in congress among republicans not just democrats.
We should not built the wall, not because of the cost but because it's not practical. Even the most optimistic estimates put the time to complete the wall as 15 years, but 20 would be more realistic since a republican congress has never put any money in budget for it. If you think that's nuts, remember it took 6 years to complete 500 miles of a fence and the fencing project was far simpler than the Trump wall and had much less opposition. Long before the wall is finished, immigration, national priorities, and ideology of the parties will have shifted along with funding of the wall.
When we enter into a project of this size we should be looking at the future, not the past. The number of illegal immigrants in the US have been falling since 2008. With economic improvements in Mexico, particular the growing shortage of farm labor, it is quite likely that the illegal immigration problem will solve itself.
Years ago I was listening to the radio when the host was interviewing Marlo Thomas. Marlo was a 70's actress who starred in a show and the daughter of a very famous actor by the name of Danny Thomas. She was promoting her book on advancing in life. In her book she interviewed a Doctor and her path to becoming a clinician.
The Doctor said she attended college and became a teacher. After some years of teaching, she became a little depressed about the career she chose. Her father realized something was wrong, and she told him of her thoughts. So he asked what is it she'd really like to do? She said she'd really like to be a medical doctor. So he suggested that she do just that! She told her father she was stuck in this career, and by the time she'd get out of college, she would be 42 years old. He looked at her and said "Well you're going to be 42 anyway!" On her fathers advice, she became a doctor and has never been happier.
This migrant problem isn't going away anytime soon no matter what you speculate. There will always be desperate people south of us, gang members who want to get in, terrorists looking for an easy way to sneak in, and people that want to take our money and send it back home. 10 or 15 years? It's going to be 10 or 15 years no matter what we do, so there is no time like the present to get started. Because every year that wall is being built, our border becomes more secure.