Apparently, 3-1/2 miles of new border wall near San Diego will cost at least $57 million, or $16.3 million a mile (or a bit over $3000 per foot). For comparison, the 350 mile long Maginot line cost France about $150 million in the 1930s, or about $2.3 Billion in today's dollars. This puts the cost of the Maginot line, underground tunnels, bunkers, gun emplacements, and all, at $6.5 million current dollars per mile. Of course, the Maginot line was not built as a continuous wall to catch individual infiltrators, but on the other hand the San Diego wall is (presumably) not being built 30 kilometers deep with layered emplacements to handle massed tank and artillery attacks.
It could be worse for taxpayers - they could be laying railroad track instead of building a wall, since that costs about $96 million per mile here in Phoenix.
I can't wait for those huge administration cost savings that are promised from nationalizing health care.
Update: I just thought of one other comparison- like the Maginot line, at least one end of this San Diego wall hangs in the air, meaning it just ends hundreds of miles before the border does, allowing it to be easily flanked.