I saw DHS Secretary Nielsen being drilled by Sen Leahy on Dems fav topic on Mexico to pay. May I help with a relatively simple response. We lost estimated $40B a year to Mexico on drug traffic. A wall can easily cut down the traffic by 25%, likely more. That's savings of at least $10B. In 2 yrs the Wall will be paid for, yes, by Mexico money, and 5 times over in 10 years.
I sympathize with Trump's staff to have to defend his loose canon style. Hope this helps.