1. It will probably be on the US side. That land is private land. Even if they have eminent domain, they still have to pay the landowner.
2. Have they figured that out in the cost of the wall?
3. Then, there is a bigger pipeline called the Trans-Pecos Pipeline that will carry gas from West Texas to Mexico, to be sold there. It takes millions of gallons of water to extract the natural gas. Who will pay for the water from the Texas aquifers? How will the wall work over the huge pipeline? The pipes are bigger than those for the Keystone Pipeline.
4. There are other pipelines going from the US to Mexico.