Because maybe, it migth NOT be a non-sequitor. Bauilding and maintaining 5,000 miles worth of wall is going to be expensive. It's not a one-time expense. People still have to man it and patrol it.
How does it get paid for?
RWA
Remember the Berlin War -- people would be shot going over the wall but they did it anyways. What makes anyone think that building a wall would keep people out? All they would have to do is get to Canada and cross over from Canada in any number of places.
Cost was not the reason the Berlin wall was brought up - and you know it.
As far as paying for one is concerned, if the Isrealis can afford it so can we.
I think devoting the $1700 per yer per illegal for education & medical we currently are paying would be a great start.