"You can pretty much predict what the results of the study are going to be by who is doing it," said Jeffrey S. Passel, a Pew Hispanic Center demographer who helped write a 2006 study on the tax contributions of immigrants in the Washington area. "There really is not an accepted methodology for assessing the costs in particular and the benefits in general."
Simply asking the question poses a philosophical debate, he said. "It's not the kind of question we ask of anybody else. No one asks: What is the benefit of society to having babies? So there are lots of ways to look at this."
In fact, determining how many illegal immigrants live in the United States is a mathematical puzzle. While the most-often cited estimate is 12 million, figures range from 7 million to 20 million.
"It's really easy when you don't know the number of illegal immigrants to come up with a wide range of the impact," said Randy Capps, a researcher at the nonpartisan Urban Institute in Washington.
Some studies include the citizen children of illegal immigrants in calculations, noting that they are entitled to public benefits such as welfare, food stamps and Medicaid, even when their illegal immigrant parents are not. These reports suggest that immigrants cost the government more in services than they contribute in taxes.
"If you want to make illegals look good in the calculation, you don't count the costs of their children and you ignore the population-based costs such as the wear and tear on roads and bridges," said Steven A. Camarota, director of research at the Center for Immigration Studies, a Washington think tank that favors restricting immigration, both legal and illegal.
In a report last year, Camarota estimated that 39,000 illegal immigrants ages 5 to 17 are enrolled in Maryland public schools. National averages put the cost per pupil at $10,000, he said. Add the citizen children of illegal immigrants, and the figure rises to 57,000, or 6 percent of total enrollment, he said.
Other observers agree that education is the largest cost of illegal immigrants. But they argue that the issue of whether to educate children is not a simple price analysis, but a question of societal values. It's also the law. The U.S. Supreme Court has ruled that states must educate illegal immigrants.
"It may be one thing when you are talking about health care and corrections, but when you talk about investing in childrens' education, you can make the case that the dollars you spend will contribute to the economy in the long run," said Capps, with the Urban Institute.
