As explained on Last Week Tonight with John Oliver, Trump's border wall will cost approximately $25 billion. The wall would not be able to keep out most illegal immigrants and prevent drug trafficking. It would harm wildlife, force property owners to cede their land to the U.S. government, etc. The Mexican Government has staunchly refused to pay for any such border wall, and such a wall would alienate Mexico, the 3rd largest trade partner of the U.S.
John Oliver proposed a cheaper alternative to the wall. Instead we get the Mexican government to buy every U.S. citizen a waffle iron. It would cost one billion less and it would not have any of the negative side affects of the wall while having an equal affect on immigration, and Mexico would be more willing to pay for this plan than a wall.