http://www.nytimes.com/2011/04/05/opinion/05tue3.html?src=recg

Every day, a bat eats much of its body weight in insects, many of
them harmful to crops. A group of scientists led by Thomas Kunz at Boston
University calculated how much more money cotton farmers in one region of Texas
would spend on pesticides if bats weren’t present. Extrapolating from those
numbers, they estimated that bats save American farmers somewhere between $3.7
billion and $54 billion a year, most likely about $22.9 billion.