I don’t know how the money will be obtained from Mexico.
I think there will be a problem taxing remittances because there are so many ways to go around Western Union. There is talk of taking the funds on deposit and assets of a major convicted drug dealer. That would be good.
My thought is that President Trump will make the Mexican government an offer they can’t refuse. I don’t know precisely what that would be but it will be such they will find accepting the offer to fork up some money, border guards,and who knows what else better than the alternative.
There will be a deal.
The continuing cost of the wall, ie staffing and maintenance etc., will need to be addressed at some point as well.
Unfortunately this would not be a one time expense, it will need to be a budgeted line item in perpetuity for it to remain an effective barrier in the future.