President Donald Trump wants to build a wall on the southern border of the US. It’s going to be expensive.

That’s about specific as it gets with the White House’s proposal to build roughly 2,000 miles of walls and fences across the southern border.

An initial estimate floated by the administration pegged the cost at $12 billion. But a recent report put together by Democrats on the Senate Committee on Homeland Security and Governmental Affairs estimated the price tag for the wall and patrolling technology at a whopping $70 billion — more than four times Trump’s initial figure.

So far, Trump’s administration has requested $3 billion from Congress in extra Department of Homeland Security funding through September — expected to be used to hire border and Immigration and Customs Enforcement agents, expanding detention and deportation, and beginning work on the wall. The 2018 budget proposal has an additional $2.6 billion for wall construction. Office of Management and Budget Director Mick Mulvaney said the president would request a total of $4.1 billion between fiscal years 2017 and 2018 for the wall, which would be only a fraction of its projected cost.

Estimates so far have varied, with $12 billion (the administration’s suggestion) at the low end and $15 billion (estimated by House and Senate leadership in January) as a more reasonable estimate. Senate Democrats’ new $70 billion estimate represents almost double DHS’s yearly budget.

With so many unknowns in the building of the wall, it’s impossible to say whose price is right.

“I mean, I don’t know what it will be made of, I don’t know how high it will be, I don’t know if it’s going to have solar panels on each side and what the one side’s going to look like and how it’s going to be painted — I have no idea,” DHS Secretary John Kelly recently testified in the Senate. “So I can’t give you any type of an estimate.”

What’s clear is that Trump’s suggested price tag was a serious lowball. And this latest estimate will likely elevate the already skeptical voices on both sides of the aisle in Congress, wary of funding a vague project with unclear outcomes.

The wall was always going to be super expensive

Put simply, building a wall, the way the president describes it, is really expensive.

When the original Secure Fence Act was passed, the 2006 legislation that called on DHS to build up roughly 700 miles of border security, Congress estimated the whole project would cost roughly $50 billion over 25 years.

It’s not just the price of materials and labor. This is a cost that will have to incorporate legal fights — kicking people off their privately owned land — and difficult terrain, including the Rio Grande river and canyons further west. Accommodating the geography will only hike up the spending.

In border towns in southern Texas, like McAllen, city leaders are advocating for levees instead of fencing — a request that city leaders along the more than 1,000 miles of river have been making for years. The idea is that levees would help mitigate flooding and provide added security. But as David Aguilar, a former deputy commissioner of US Customs and Border Protection, admitted in a Senate hearing on the wall in early April, prioritizing these demands would be much more expensive than building a typical land barrier in Yuma, Arizona.

“You have to decide where you are going to put your resources,” David Danelo, a national security expert with the Foreign Policy Research Center, told Vox. “It’s going to be ugly. There’s going to be cost overruns when it comes to South Texas.”

DHS isn’t waiting for Congress to appropriate money to move forward. It’s already in the process of selecting contractors to build wall prototypes — 30-foot segments of wall, in concrete or some other material — as a way of figuring out what a successful wall might look like and how much it could cost. But the fact that the department is already asking Congress for billions demonstrates that there’s no price too high — and that it’s hoping to ask Congress to pay for the wall in installments, even without anyone knowing how much, in total, the project will cost.

As of now, the strategy — fund incrementally now, figure out the rest later — has raised some eyebrows across party lines in Congress. Democrats have threatened to take the government to the brink of a partial shutdown, if necessary, to block wall funding. With no detailed plan from the White House, some Republicans aren’t a sure bet for support either.

Paying for the wall is shaping up to be a losing battle

We know Mexico isn’t going to pay for the wall first.

“I never said they're gonna pay from the start,” Trump told ABC’s David Muir in his first major TV interview as president. “I said Mexico will pay for the wall. … I wanna start the wall immediately. Every supporter I have — I have had so many people calling and tweeting and — and writing letters saying they're so happy about it. I wanna start the wall. We will be reimbursed for the wall.”

But in Washington, even the notion that Mexico will eventually pay for the wall has become laughable. Most Republican lawmakers have stopped playing along. Mexico’s leadership is adamant that it will not pay for the wall, and in early April, Secretary of State Rex Tillerson told ABC’s George Stephanopoulos that he has "had no conversation" with Mexico's foreign minister about the subject.

At the end of the day, everyone is well aware that if the wall is going to be built, it will have to be funded through Congress first, with taxpayer dollars. What no one knows is just how much taxpayers will be on the hook for. Already Democrats have put their foot down, warning Republicans that any attempt to fund the border wall would lead to government shutdown. According to reports from Capitol Hill, Republicans, who are eager to avoid that outcome, have been trying to avoid the subject of wall funding in recent government spending negotiations.

The one possibility for Trump to have his way with Mexico, with a border adjustment tax, is also shaping up to be a nonstarter in Congress. Currently being floated by some House Republicans, this reform would tax imports and exempt exports (Vox’s Dylan Matthews explains this in greater detail), which scores as raising money because the US currently imports more than it exports; the revenue raised far outstrips the cost of the wall.

But, confusingly, Trump has both said he is interested in a “border adjustment” tax and rejected the idea of what a “border adjustment” tax actually is. And it’s unlikely the proposal will even make it through the House, let alone the Senate. Members of the House Freedom Caucus — the conservative faction of the party that successfully stopped the Republican health care plan — have already expressed skepticism.

Whether via border adjustment or direct appropriations, the road to funding the wall runs through Congress. And it’s clearer than ever, given Senate Democrats’ eyebrow-raising estimate, that they (and likely also Republicans) have no intention of giving Trump money to build it.