It's already paid for. Congress allocated the funds over a decade ago. It was foot dragging from the White House that kept the wall/fence from being completed in a timely manner.
Not the type of wall that Trump wants. Also, even with Trump's wall I read somewhere that there will be gaps in it and the cost as his wall has been proposed will exceed that already agreed upon by several magnitudes. Do you seriously believe a wall would work. Did not work for the Chinese. Has not worked for the Israelis, did not work for the Warsaw Pact countries. It is simply a feel good political venture and will not solve anything. it will run up the national debt significantly.