The United States apparently pays a lot for housing for illegal aliens in the United States. It might be a better investment to train El Salvadoran carpenters and to pour concrete dome homes and train builders for aluminum panel homes in El Salvador.
https://www.npr.org/2019/05/10/722173775/proposed-rule-could-evict-55-000-children-from-subsidized-housing H.U.D. proposes law to make it illegal allow illegal aliens in U.S. public housing
https://explorer.usaid.gov/cd/SLV U.S. foreign aid
The U.S. spent 118 million dollars in foreign aid to El Salvador in a recent year, and the Wall requires billions.Training El Salvadoran security personnel in the United States and sending them back to guard neighborhoods in El Salvador might be worthwhile, as might solar panel installation and water purification.
Plainly the U.S. government cannot reasonably break the law and pay for public housing for people that have criminally entered the United States. It can however build housing in nations that send hundreds of thousands of people here because of job, housing and security conditions in their home country.
It is necessary to remedy the problem of illegal entry with increased border security and improving the condition of the nations that send people north. They are two sides of the same coin of chaos and anomie.