HIDALGO, Texas - Perched 20 feet above a South Texas cabbage field in a telephone booth-sized capsule, a National Guardsman passes a moonlit Sunday night with a gun strapped to his hip, peering through heat detector lenses into an adjacent orange grove.

Deployment of 1,200 National Guard soldiers for one year: $110 million.

This same night, farther west on the border, a haunting whistle blasts through the predawn quiet as a mile-long train groans to a heavy stop halfway across a Rio Grande River bridge. In a ritual performed nightly, a Customs and Border Protection agent unlocks a gate, a railroad policeman slides the heavy doors open, and they both wave flashlight beams under, over and in between the loads of cars, electronics and produce, before they pass through an X-ray machine searching for hidden people or drugs.

One rail cargo x-ray screening machine: $1.75 million.

On this night in southern Arizona, a screener examining tractor-trailer loads of charcoal spots something odd and asks for a closer look. Drug sniffing dogs bark. He finds 8,000 pounds of baled marijuana in several trucks.

Customs and Border Protection officer average annual salary: $75,000. Drug-sniffing dog: $4,500.

As Congress debates border funding and as governors demand more assistance, The Associated Press has investigated what taxpayers spend securing the U.S.-Mexico border.

The price tag, until now, has not been public. But AP, using White House budgets, reports obtained through Freedom of Information Act requests and congressional transcripts, tallied it all up: $90 billion in 10 years.

For taxpayers footing this bill, the returns have been mixed: fewer illegal immigrants but little impact on the terrorism issue, and certainly no stoppage of the drug supply.

The Sept. 11, 2001, terrorists didn't come from Mexico, but the attacks led politicians to re-examine border security. Ten days later, President George W. Bush announced a new Department of Homeland Security, with tasks including the security of the nation's porous southern border.

Over the next 10 years, annual border spending tripled as the U.S. built an unprecedented network along the 1,900-mile border with Mexico: 165 truck and train X-ray machines; 650 miles of heavy duty fencing and sheer concrete walls; twice as many law enforcement officers along the entire stretch, and a small fleet of Predator drones. Also, remote surveillance cameras, thermal imaging devices and partially buried ground sensors that sound an alarm back at headquarters if someone steps on one in the desert.

"Our obligation to secure our borders involves a responsibility to do so in the most cost effective way possible, and we recognize that there is no 'one size fits all' solution to meet our border security needs," said Department of Homeland Security spokesman Matthew Chandler.

Over the years, the goals of the border security measures have shifted.

Early concerns that terrorists could sneak weapons into the U.S. from Mexico were later overshadowed by worries about violent drug cartels slaughtering people across the Rio Grande. As the U.S. economy faltered, preventing illegal immigrants from sneaking north for jobs became the focus.

"Border security is no longer just about responding to 9/11. It became very much a part of the immigration debate," said Jena Baker McNeill, homeland security policy analyst at The Heritage Foundation, a conservative think tank based in Washington, D.C.

Indeed, stopping immigrants at the border has become a bargaining tool for the last two administrations with Congress - fences and guards in exchange for reforming immigration laws, she said.

The buildup has dramatically reduced illegal immigration. Ten years ago, border agents caught 1.6 million illegal immigrants in one year. Last year they caught just 463,000. The drop is attributed in part to the U.S. recession which decreased jobs here, but it's also an indication, according to federal officials, that fewer people are attempting to illegally cross the border.

But the spending has not worked to stop the flow of illegal drugs. Last year, border guards seized a record 254,000 pounds of cocaine, 3.6 million pounds of marijuana, and 4,200 pounds of heroin. In response, Mexico's cartel bosses simply sent more: trainloads of marijuana, cocaine stuffed in fenders and dashboards, heroin packed into young men's shoes.

An estimated 660,000 pounds of cocaine, 44,000 pounds of heroin and 220,000 pounds of methamphetamine are on American streets in a given year, according to the Office of National Drug Control Policy. A fraction of that amount is seized at the border, a small operating cost for Mexico's drug lords, who will reap an estimated $25 billion this year from their U.S. sales.

Last month, a Justice Department study reviewing the total cost of illicit drug use in the U.S., using cost-of-illness studies, federal crime and caseload statistics, and economic models, came up with a figure of $193 billion per year.

"You can't ever seal the border. You can never stop anything 100 percent. As long as there's a market, as long as there's a profit, there will always be someone taking a chance on getting that product through," says Democratic U.S. Rep. Silvestre Reyes of Texas, a former Border Patrol director.

Despite the surge of violence just a stone's throw away - the death toll in Mexico's crackdown on cartels is more than 35,000 - the Obama administration reports communities on the U.S. side of the border enjoy relative peace. Nor have terrorists typically crossed the border to enter the U.S., officials note.

Still, Rep. Michael McCaul, a Texas Republican, warns against complacency.

"There is a disagreement about the definition of spillover violence and the extent of such violence, but there should be no disagreement about the threat we face and what will happen if this Administration continues to downplay the threat," he said. "So what should we do? For starters we should get out of our foxholes and lean forward against this growing threat. If we don't, the cartels will eventually attempt to take over our cities."

If Department of Homeland Security Secretary Janet Napolitano could talk to Mexico's drug cartel bosses, here's what she says she would tell them: "Don't even think about bringing your violence and tactics across this border. You will be met by an overwhelming response."

And if she could talk to would-be illegal immigrants, she'd say this: "There are more Border Patrol agents on that border than ever before. There are more customs officials. There is more technology. Do not throw in your lot with the cartels or the criminal organizations because the likelihood of getting caught, and the consequences of doing so are higher than ever before."

For 2012, the Obama administration's record high budget for border security proposes an additional $242 million to pay for high tech watch towers and movable screeners along the border, $229 million to raise border agents' pay, and $184 million to identify and deport criminal aliens in state prisons and local jails. That's on top of about $14 billion to support the ongoing infrastructure.

Over the years, budget allocations tell a story of a shifting border policy.

In 2002, as post-9/11 security checks created four-hour waits on the border, the Bush Administration sought $380 million to construct a state-of-the-art entry and exit visa system.

In 2006, the federal government ended an immigration "catch and release" policy in which local police had been releasing illegal immigrants if they hadn't committed a local crime. Now they would be turned over to feds and face immigration charges. That year taxpayers spent $327 million for 4,000 new beds to hold the suspected illegal immigrants until they could be legally processed.

This January, the Obama administration dumped SBInet, an attempt to install a high-tech "virtual" border fence project that cost taxpayers nearly $1 billion but did little to improve security.

"From the start, SBInet's one-size-fits-all approach was unrealistic," said Connecticut Sen. Joe Lieberman, the chairman of the Homeland Security and Governmental Affairs Committee. "The department's decision to use technology based on the particular security needs of each segment of the border is a far wiser approach, and I hope it will be more cost effective."