Federal spending on border security is at an all-time high—and it would get even higher under the Gang of Eight’s new plan. The Senate immigration proposal, released last week, would allocate $4.5 billion in the next five years to tighten control of U.S. borders.

The U.S. spent nearly $18 billion dollars on immigration enforcement agencies last fiscal year, more than all other law enforcement agencies combined.

Where would another $4.5 billion go? Here’s a closer look at what is being proposed, and how the government has spent (and often wasted) border money in recent years.

More border agents

The proposal calls for an additional 3,500 U.S. Customs and Border Protection officers. In FY 2012, the department employed 21,790 officers, up 10 percent from 2008. The bill would also add an unspecified number of Border Patrol agents, whose ranks have skyrocketed from just over 4,000 in 1993 to more than 21,000 today.

A 2011 investigation by The Center for Investigative Reporting and the Los Angeles Times showed how hurried hiring by the border agency affected screening standards and led to an increase in corruption. From 2006 to 2011, the number of investigations of customs employees charged with fraud more than tripled. Since 2004, 147 agency employees have been charged with or convicted of corruption-related offenses.

More drones

The bill requires buying as many “unmanned aerial systems” (also known as drones) as needed to have 24/7 surveillance of the Southwest border. The U.S. has already purchased 10 border drones, which cost $18 million a piece and roughly $3,000 an hour to operate.

Many question whether the current border drones are worth the investment. According to a report from the Customs and Border Protection agency, drones led to 143 arrests and the recovery of 66,000 pounds of drugs in 2012. As news outlet Fronteras calculated, “that’s less than 3 percent of all drugs seized by border agents last year, and less than 0.04 percent of the 365,000 would-be illegal border crossers caught by agents.”

In May 2012, a report by the Department of Homeland Security’s Office of the Inspector General found the U.S. didn’t have enough manpower or money to effectively operate the drones they already have. The department overshot its maintenance and operational budget by over $25 million. Drones had only flown for 30 percent of the time they were supposed to be in the air.

More fencing

Another $1.5 billion would be allocated to expand the 651 miles of fencing along the Southwest border. "I think what we would do if the bill passes," Homeland Security Secretary Janet Napolitano said in a Senate hearing, "is go back and look at the type of fencing we have and say, ‘Do we want to make it triple what it is or taller?’ — or something of that sort."

More phones and radios

Remote areas along the Southwest border can have spotty cell coverage, posing a risk to border guards in an emergency. A two-year grant would provide more funding for satellite phones and radios for border staff to contact 911, local police and federal agencies.

The bill doesn’t say anything about training guards to use the new devices. In November, we reported how DHS had spent $430 million on radios that only one surveyed employee knew how to use.

More money for local cops

Some of the new DHS funds would go toward Operation Stonegarden, a $46.6 million FEMA program benefiting local law enforcement in border states. "The funds that we are getting from Stonegarden are a godsend," a county sheriff told the Arizona Daily Star in 2009. "I think we are able to provide a lot more security, a lot more visibility."

But critics say there’s little oversight of how the money has been spent. The Star’s review of Arizona police records showed grant money was funnelled toward expensive technology and overtime pay for cops doing unrelated tasks, like crowd control at city parades.

More accountability?

As Congress considers adding billions more to the border budget, lawmakers are left with a key question: is it working? Some critics on the left say the added funding may be unnecessary, as studies suggest net migration from Mexico is now below zero. Many on the right say there still aren’t enough hard metrics to judge whether Homeland Security is doing a better job of keeping undocumented immigrants out.

DHS has pointed to the drop in the number of apprehensions as a sign U.S. borders are stronger now than ever before. But critics say it’s a flawed way of judging whether the billions spent on border security are worth it. That number could mean fewer undocumented immigrants are attempting to cross the border, or that fewer are being arrested. The struggling U.S. economy also plays a big role in the overall drop in unauthorized immigration.

Under the new proposal, high-risk sections of the Southern border must reach a “90 percent effectiveness rate” within five years. That would be the “number of apprehensions and turn backs” divided by “the total number of illegal entries.”

If border states don’t reach the 90 percent target, a group of border state governors (or their appointees) and federally-appointed security experts would step in to draft a new plan to boost effectiveness—on which the DHS can spend up to $2 billion more. The new bill would also create a presidentially-appointed DHS Task Force to regularly review border enforcement policies.

Increased surveillance should help border agents get a better count of the total number of undocumented immigrants crossing the border, said Doris Meissner of nonpartisan think-tank the Migration Policy Institute. According to Meissner, this is the first time immigration legislation has included a specific metric to gauge whether money spent on border protection is resulting in fewer unauthorized crossings.

“The overall expectation that so much money has been invested, the government has to do better in really laying out how it assesses its effectiveness,” she said.