If enforecement of the border is currently costing us billions, and we're paying billions more to house, feed, clothe, educate and medically pay for illegals...then if the wall helps us save even half of those billions it's money well spent and eventually saved.
We wouldn't even need a wall if we'd simply enforced our existing immigration laws. We wouldnt need a wall if we eliminated free education, health care, housing and food stamps to people who come here illegally!
Meanwhile, if Mexico is paying us billions more through our new trade deal with them than they were paying us before the wall, that revenue can then be added to the savings from border enforcement and handling of illegals...and fed into future deportations and streamlining of our entire immigration activities which saves us even more money that not only pays for the wall but increases revenue to the treasury we can use to lower income taxes as Mexico pays us even more to offset our ongoing operations.
Win...win...win...Trump!