Donald Trump has made building a wall with Mexico the centerpiece of his campaign. Yet he doesn’t even list a price estimate for such a wall.

Yet America’s attempt to build a wall under the Bush administration has given us some idea about how much such an endeavor would cost. And it’s not just the price tag that will undercut public support for the wall. Republicans, Independents and Democrats have come to recognize that such a plan wouldn’t work.

Trump’s campaign website provides positions on only seven issues. For the sake of perspective, Bernie Sanders posted 34 issue positions online and Hillary Clinton gave voters campaign stands on 31 issues on her website. Even New Mexico Gov. Gary Johnson, who just clinched the Libertarian Party nomination, gives us 12 issue positions.

Nowhere on Trump’s position on immigration reform does he list the cost of building his wall. Given that this is the foundation of his entire campaign, it’s a bit curious to leave out this detail. All he says is that the cost of building the wall pales in comparison to the cost of illegal immigration to this country, and that Mexico will pay for the cost of building the wall. Despite all the threats of tariffs and withdrawing visas and trying to stamp out remittances, there’s not a single indication that Mexico will provide a peso for the wall.

Actually, Trump is hardly the first person to try and build a wall with Mexico. George W. Bush tried to construct one in 2006, the “Safe Fence Act,” costing nearly $3 million per mile for the fence.

Bush tried to cover the rest with cameras, the infamous “virtual fence.” Believe it or not, President Barack Obama bought into the idea, and outlined a plan to continue the construction.

But plans to continue project was abandoned in 2010 amid a scathing report from the nonpartisan Congressional Government Accountability Office, which showed that the plan clearly wasn’t working. One report on such a fence found that only 1 percent of alarms led to arrests, according to research by Spencer Hsu with the Washington Post. The project was criticized by Democrats, Independents and even Republicans.

Conservative GOP Congressman Michael McCaul of Texas, the chair of the House committee on homeland security, told the right-wing publication Daily Caller that, “In our conversations with outside groups, experts and stakeholders, we learned that it would be an inefficient use of taxpayer money to complete the fence. … We are using that money to utilize other technology to create a secure border.” That quote came from CNBC, by the way.

CNBC’s Kate Drew researched the cost of building a wall, as opposed to a fence. She found that the $2.8 million per mile for a fence was artificially low because it only included fences constructed on public lands. Walls cost a lot more than fences. Drew found that the GAO found just one section of the fence in San Diego cost $16 million. And she adds that these figures don’t count what it takes to maintain the wall, as well as the cameras, personnel to maintain it, private land that needs to be purchased, etc.

There’s a reason no other Republican presidential candidate embraced the plan, even though it clearly resonated with some GOP primary voters. The International Business Times reported on Jeb Bush’s trip to McAllen, Texas, where elected officials there told him that building the wall wouldn’t work.

“The cost of it is extraordinary, the terrain makes it impossible. It’s a great sound bite, but it’s not defensible in terms of a practical policy,” Bush said, according to IBT.

The American people deserve to know how much the border wall would cost to build and maintain, as well as whether or not such a wall would ever even work. Even Trump’s supporters are likely to be disappointed if he’s elected and attempts to start such a project.