While illegal crossings are impossible to count, experts look to Border Patrol apprehensions as the best indicator of migrant traffic.
For all the rhetoric about how much illegal immigration costs California and other states, economists who have studied the issue say the underlying numbers are conflicting and conflated.
And that, in fact, the costs actually do not outweigh benefits.
"There's a significant body of estimations, of literature out there, that shows there's a 'net plus' to having undocumented immigrants," says James Gerber, Ph.D. professor of economics and director of the international business program at San Diego State University. "It's not an argument for (illegal immigrants), but it's just a serious attempt to provide a numerical estimate of their physical presence."
Gerber cites a variety of studies including those conducted by the National Academy of Sciences, the Public Policy Institute and the University of California, Davis.
"The majority of (illegal immigrants) pay social Security taxes, he pointed out. "Many of them pay income taxes. They all pay sales taxes. They pay property taxes from the rents they pay for the places they rent. Many of them are property owners, in fact."
Gerber's remarks came in response to recent statements made by supporters of Arizona's polarizing new immigration law.
The vice president of Local 1613 of the National Border Patrol Council, representing Border Patrol agents in the San Diego and Imperial Counties region, praised the Arizona law during a Thursday news conference.
"There are millions of illegal aliens in this country, and until we start taking steps like this, we'll continue to be overrun," said agent Shawn Moran. "The infrastructure of our state is going to be worn down. Hospitals are going to close down ... I hope (the law) stands up to all the legal challenges that occur in Arizona."
Addressing an April 24 Tea Party gathering in Ramona, U.S. Rep. Duncan D. Hunter, R-El Cajon, warned that "California's going under ... between ten and twenty billion dollars in this state (is what) we spend in illegal immigration. It's health services, it's education, it's jails."
While skeptical of those figures, Gerber says they have a galvanizing effect on public opinion.
"It's quite common when we have an economic downturn to look for scapegoats," he said. "Obviously, immigrants had nothing to do with the financial crisis, which has had a terrible impact on the state of California and other states around the nation."
Gerber says the supply of illegal workers tends to hold down labor costs -- and thus consumer costs -- in especially in agriculture, construction and tourism.
"Locally, for example, tourism in San Diego could not exist in anything near its present form without undocumented immigrants," he said. "They come to find jobs, to try to better their futures to create opportunities for their children. So when they come, they add an element of dynamism, energy, and entrepreneurial activity to the economy."
Whatever the purported benefits, Uncle Sam has still has little tolerance for employers of illegal immigrants.
The government is now seeking the forfeiture a venerable restaurant and bakery in San Diego, the French Gourmet, whose owner faces criminal charges stemming from a 2008 Immigration and Customs Enforcement raid.