Building the wall is not that big of a deal.
Deporting 11 million is not that big of a deal.
But there is no way in hell Mexico is going to pay for the wall.
I don’t remember anyone saying Mexico would be willing to pay for a fence or wall. It is possible to make it happen though. We give all sorts of aid and assistance to Mexico, their businesses, and citizens. It is possible to make a deal with them they cannot refuse.
Maybe the Mexican government won't, directly, but their citizens in this country can by paying a tax on remittances from this country to Mexico.