The chant “build a wall!” was a common refrain at Donald Trump’s rallies on the campaign trail. Many of his supporters voted for him based on his promise to build a wall along the U.S.-Mexico border, and now the president is trying to make it happen for the price of $8-12 billion dollars.

That’s just not true, says Konstatin Kakaes, a reporter and fellow at New America, the Washington, D.C. think tank that recently took an in-depth look into the proposed wall for the MIT Technology Review.

Here’s what he learned:

What would the wall look like?

On March 14, the Department of Homeland Security Customs & Border Patrol put out a request for proposals to build the wall. The request suggests the administration wants a structure that’s at least 30 feet tall, and meets standards for aesthetics and resistance to climbing, tampering, and damage.

What the request does not say, according to Kakaes, is how deep they would want to dig to prevent tunneling, and how long they would want to build the wall. Length, he says, “is a huge driving factor in the cost because the 700 miles that were already fortified under the Bush administration? Those were the 700 easiest, most logical miles to build on.”

The actual cost of the wall

President Donald Trump has estimated that the cost of a border wall would fall between $8 billion and $12 billion. The real cost of a 30-foot high wall along the 2,000 mile border, Kakaes says, would be closer to $70 billion.

Kakaes spoke with engineers and construction experts to develop a budget for the project, considering factors like the cost of labor, the cost of materials, and ancillary costs.

“The numbers are by intention a floor and not a ceiling. There are many factors that we deliberately did not account for,” Kakaes said, such as the cost of acquiring land, which may also come with legal challenges.

The large gap between the estimates

“I have not spoken to anybody in the administration about this,” Kakaes said. “But I think the general reaction of people I've spoken to on Capitol Hill about this is that President Trump's estimate is a deliberate low-balling of the numbers.”

Is the wall needed?

Some say that a wall is unnecessary, pointing to statistics that show fewer people are being arrested trying to cross the border illegally. Kakaes goes further, saying that even if a wall was built, it wouldn’t work.

The border between the U.S. and Mexico is huge, Kakaes said. According to the federal Custom and Border Patrol, 180 million people crossed back and forth between the U.S. and Mexico in 2015.

“If you don't realize how big it is you don't realize that an attempt to control every single person's entry and exit to the country with absolute certainty is simply not possible," Kakaes added. "It drives you into a sort of authoritarianism.”

Other ways to secure the border

As part of the fence built on the border, the Bush administration implemented  a “virtual fence"—predator drones that continue to fly along the border as additional surveillance. But according to reports from the Government Accountability Office, Kakaes says, “the drones cost a great deal of money to operate without having a large effect.”

Ultimately, people crossing the border are driven by the demand for labor in the American economy. Kakaes suggests building up the economies of Mexico and other Central American countries to help reduce the flow of people trying to cross the border without papers.

“The cost effectiveness of interventions of that sort just dwarfs it," he said. "Infrastructure is an extremely expensive thing to do. It always has been and always will be.”