The military has been flying Operation Nobel Eagle missions for more than 12 years, but efforts to figure out how much defending U.S. air space has cost have failed.
The mission is overseen by the North American Aerospace Defense Command in Colorado Springs.
NORAD leaders say figuring out the cost is a complicated business made more complicated by the fact that several branches of the military are defending the skies.
The operation started on Sept. 11, 2001, after hijacked planes hit their targets in Washington and New York.
Every day, pilots and aircraft at 16 bases are assigned to protect high-value targets in the United States and Canada.
The program has flown more than 70,000 sorties.
But since 2009, when Congress asked for a price tag for the program, Pentagon accountants have been unable to determine how much it costs.
Congress got involved after Pentagon planners decided to take two air bases off 24-hour alert status, a move they estimated would save $73 million over five years.
The Government Accountability Office asked for an accounting, but reported last month that they were frustrated in their effort.
Auditors said the Pentagon "has not yet reported the comprehensive cost of the mission."
Steven "Stretch" Armstrong, a civilian who works in NORAD's operations section, said the biggest issue with determining cost has to do with how the Pentagon pays for flying planes.
Units are given a set number of flying hours to train for a variety of missions, from combat in Afghanistan to defending the skies at home.
Pilots don't punch a stopwatch to determine how much time was spent flying for each operation because the training is nearly the same.
"They're responsible for multiple, multiple missions," Armstrong explained.
While the financial accounting may be tangled, NORAD's role has been praised.
Planners at the Peterson Air Force Base command have pioneered computer techniques to determine how many planes and bases are needed for the mission through risk analysis.
A computer model shows the military can deal with threats with fewer resources.
"Our risk assessment is based on threats and response," Armstrong said.
NORAD officials say the operation remains crucial to American security.
Armstrong said terrorists are constantly working on new methods to use planes in their attack plans.
"The threat has morphed to respond to the mitigation measures we have put in place," he said.
Since 9/11 the United States has faced no similar attack, though terrorists have attempted to blow up airliners with improvised bombs.
"That doesn't mean they're not trying to figure it out," Armstrong said.
And the Pentagon will keep trying to figure out how much Operation Noble Eagle costs taxpayers.
Top Defense Department officials promised the GAO that they'll have another look at the books.