Answer: The exact amount the U.S. has spent in Libya to-date has not been released. Yet, according to the Pentagon, the cost through July 31 was $896 million. If costs have been sustained (about $6.6 million a day) then the total now would be $1.05 billion.
Micah Zenko at the Council on Foreign Relations runs the numbers from available data and comes up with $1.1 billion.
To compare, the U.S. was spending $6.2 billion and $3.5 billion each month in Afghanistan and Iraq, respectively, as of the end of April.