A great article came to my attention, discussing what constitutes "acceptable" and "unacceptable" risks. It is found at http://www.foreignaffairs.com/articles/66186/john-mueller-and-mark-g-stewart/hardly-existential?page=show
Professor Mueller surveys the regulatory practices of developed countries and finds there is general agreement on what constitutes an unacceptable risk. A death rate of 1 in 10,000 per year is generally considered unacceptable, as in a 1980 U.S. Supreme Court case in which the death rate from workers inhaling gasoline vapors was deemed to be unacceptable at 1 in 40,000.
Similarly, a death rate of less than 1 in a million is considered acceptable.
The risk from terrorism is way lower than what is considered in other areas as acceptable, so obviously the vast funds spent are out of line. Mueller states: "Compared with dying at the hands of a terrorist, Americans are twice as likely to perish in a natural disaster and nearly a thousand times more likely to be killed in some type of accident."
To get up to the upper border of what is unacceptable, which is 1 in 100,00 deaths, even after accepting the dubious DHS proposition that lives lost to terrorism are twice as valuable as lives lost otherwise, Mueller states that "the number of fatalities from terrorist attacks in the United States and Canada would have to increase 35-fold; in Great Britain (excluding Northern Ireland), more than 50-fold; and in Australia, more than 70-fold. For the United States, this would mean experiencing attacks on the scale of 9/11 at least once a year, or 18 Oklahoma City bombings every year."