Even having Mexico pay for the wall means our products from Mexico will be more expensive. You see, Mexico will have to tax to get that money and that tax will need to be compensated in some way.
Get it?
Stupid answer...
Mexico has a trillion dollar economy. Even if they decide to raise taxes on their whole economy to get the $20 billion for the fence, that would not increase the costs of their exports to us by $20 billion. At most it would be a small fraction of that. Additionally they may not want to increase taxes on US exports at all since that would end up reducing the exports and cutting their noses to spite their faces. Or they may choose to not increase taxes at all but reduce expenses in some other program.
Get it??