According to the U.S. Census Bureau report the median child support payment in the U.S. is $280 per child a month.
Of course that doesn’t mean that the money was actually paid. The census bureau also reports that only about half of parents owed child support in 2007 received the full amount. About a third received some of the child support, and about a quarter received none of the child support owed.
For further information, check this link: http://www.census.gov/newsroom/releases/archives/children/cb09-170.html