If you extended the fence from sea to shining sea, and didn't add a bunch of patrols along its entire length, you'd probably see illegal immigration drop 25%. (Among other things, an unpatrolled fence invites people to cut holes in it.)
75% of a calamity is still a calamity.
There is no cheap, easy solution to illegal immigration, aside from:
1. Hiring a bunch of illegals to build a fence from sea to shining sea. (Sorry, you can't do a "cheap" fence at union scale rates.)
2. Drafting every able-bodied young American male for two years to patrol said fence. Pay 'em at the going rate for a US Army private.
3. Drafting every American woman of child-bearing age to have the next generation of border guards.
Just think: we'll need to make unauthorized infertility a court-martial offense.