Concrete is over 100 dollars a yard. A 20 wall will require a deeper footing than normal. So add that to the 20 feet. Say 26 ft. At 8 inches thick and however many miles it will be. The cost would be way more than its worth.
It wouldn't be solid concrete though. I get your point that it will be expensive, but it's definitely doable and probably cheaper than all the deportation and investigation into illegals. :{
You know how Obama calls it Obamacare? Will it be Trumpcare if he wins? If so, people can call it pulling the Trumpcare on illness. lol I can see him marketing it now... :{
We could implant sensors in the wall though that would alert strategically placed stations that there is a breach or disturbance. It's completely feasible. :{