Estimates are currently coming in at over $25 billion dollars. Of course, Trump says "Mexico is going to pay for it" and Mexico is saying "not us". The entire Department of Transportation expenditures in 2015 were $76 billion. That budget goes to shoring up the bridges on the interstate system, the paving of the interstates, the maintenance of ports and airports. The FAA, the NHSA, FHWA and a host of other important agencies are all part of the Department of Transportation. The entire budget of the USCIS was around $3 billion, and the parent Department of Homeland Security was $41 billion.
If we added the total budgets of the Department of Transportation and the Department of Homeland Security together the available budget would be about $120 billion. So, if Trump were to build his wall without Mexico kicking in the $25 billion needed, it would fall on either of these budgets to cover the cost. If we took what amounts to 20% cut to their budgets we would have to pull that money from the TSA, the repair of bridges, the running of the Coast Guard, the inspection of railroad track and signals, the inspection of cargo coming into the US, the processing of immigration, and on and on and on. The country with a 20% cut in those core services could come to a virtual standstill all for a wall.
This does not include all the legal hurdles involved in doing environmental impact studies, eminent domain, engineering in difficult areas, etc. It may be cheaper and easier to go to Mars.
We have to approach this type of thinking in a rational way. Dollars, contrary to what Trump says "because you print the money", do not grow on trees. It is time for us to start taking a hard look at some of his outrageous proposals and what amounts to the real cost.
Maybe a smaller Spinal Tap version of the wall would work ;)