What would be the bad and good effects both economically and politically on the United States if, according to the suggestion of isolationism, we just “built a wall” around the country and didn’t deal with outsiders?

President Trump's plan to build a wall between the United States and Mexico has been shunned by a majority of Americans. A Gallup poll released in February 2019 found that 60 percent of Americans were against the wall idea, a slight increase from seven months earlier. Only 40 percent said they favored building the wall. Here's a look at the pros and cons of this project.
Pros: Secure Border
Even though most Americans oppose the wall, a majority favors boosting security. Proponents of the wall say that it will reduce drug smuggling and illegal immigrants entering the country. They also believe it will reduce lost revenue that has ended up in the hands of illegal immigrants supposedly exploiting the system. Although illegal immigration has declined dramatically since the 1990s, a more secure border will encourage immigrants to follow the legal process to become US citizens.
Cons: High Costs and Trade Barriers
The cost of building the wall has risen since Trump began talking about it during the 2016 campaign. Although the cost in early 2019 has been estimated at $7 billion, when factoring in long-term maintenance costs, the grand total in several years will likely exceed $50 billion. A key reason for the high cost is that the wall will span 2,000 miles. A wall dividing the two countries could have a negative effect on trade and US-Mexico relations. Mexican President Nieto has affirmed that his country will not pay for the wall.
https://news.gallup.com/poll/246455/solid-majority-opposes-new-construction-border-wall.aspx

https://www.thoughtco.com/mexico-border-fence-pros-and-cons-1951541