Only about half of the illegals enter through the southern boarder, the other half come in my air, by boat or the Canadian boarder. A wall would not solve the problem and is probably the least cost-effective way to attack the issue.
Surveillance drones are now being employed which help quite a bit. One thing I have never understood, if the DOD has satellites that can read the license plate on a vehicle parked in Baghdad, why can't the same technology be employed to monitor and scan the southern boarder for illegal entry?