I think the idea of building a giant wall across our southern border doesn't make very much sense. For one thing, I simply disagree with those who think we have a serious "too many Mexicans" problem in this country. Insofar as we do have a "too much lawlessness" problem related to the large number of illegal immigrants living here, I favor some form of "earned amnesty" as the most humane and economically reasonable method of regularizing the situation. Insofar as one is interested in seriously reducing the number of illegal immigrants, the thing that's needed is not a physical barrier but a well-designed system of employer penalties. People are coming here to get jobs, and if you can make it so employers don't want to offer the jobs to illegals, people will stop coming illegally.
Meanwhile, a wall is very unlikely to be effective. The country has a coastline, people overstay visas, you can hide people in the back of trucks, etc. People build walled borders to keep their citizens locked in, not as a method of immigration control. Meanwhile, the expense will be huge -- this is a very large wall indeed that we're talking about. On the other hand, like Tyler Cowen I can't help but wonder if a large, ineffective wall might be the best possible outcome for pro-immigration people at this point. It wouldn't really work, but it would sharply diminish political pressure to "do something" about immigration. Meanwhile, as a liberal I don't really have a problem with the idea of an enormous wasteful construction project. It's like a WPA-style jobs programs. And, of course, all the building trades work just north of the border will probably attract a lot of immigrants.