Trump will in fact build a wall. Yes, it might be a fence in some places, and it might be electronic surveillance near natural barriers in other places, but it will be effective in that, when completed, it will reduce illegal immigration by a factor of 10.
As for Mexico paying for the wall, well, it already has. The Mexican peso was 18.3 to the dollar on Tuesday, and now it is 20.6 to the dollar. It has lost 11% of its value because of Trump's election. Given that we import roughly $20 billion of Mexican goods and services per year, which are actually denominated in pesos, the terms of trade have shifted in our favor by $2.2 billion per year. That seems to cover the rate of spending a wall would require.