You may already know that the child support guidelines provide a formula which produces the presumptively correct amount for child support. There are only a few variables in the formula: the income of each parent, the amount of alimony or previous child support paid, the amount of work-related child care, the amount of health insurance premiums for the children and the amount of extraordinary medical expenses.
Based on the number of children and the combined incomes of the party, there is a presumptive amount of support that those parents would need to spend in total to support the child. Where does this number come from? It is based on research on what families spend at particular income levels (although usually somewhat outdated). The USDA recently released its annual report on the cost of raising a child. The report found that a child born in 2010 will "cost" nearly $227,000 until the age of maturity, excluding the costs of college education. This amount is an average, with lower income families necessarily spending less and higher income families spending much more.
A full version of the report can be found at the USDA's website for its Center for Nutrition Policy and Promotion.