Trump can more than pay for a wall.

Trump is worth 3.1 billion. The wall will cost 8.25 billion. If Trump were to pay for the wall himself, even if he went bankrupt in the process, he would still be worth more than the wall costs. (3.1 - 8.25 = -5.15) If he went bankrupt, he'd still be 3.1 billion dollars worth of "more than the wall costs" bankrupt. 3.1 billion is more than 0, so he would still be worth more than the wall costs.