Two mystery space tourists put down a “significant deposit” with SpaceX to take a round-trip around the Moon, CEO Elon Musk announced yesterday. Musk didn’t say much about the two unidentified passengers, let alone how much money they’re shelling out for their Moon voyage. Turns out, it’s remarkably difficult to guess the costs of human spaceflight.

“There’s not just a line item that says, ‘Send this person to space.’”

That’s because, unsurprisingly, there’s a lot that goes into launching someone into space. There are the obvious costs: the spacecraft, the rocket, and the fuel. But then there are the less obvious, just as key, costs: the years and equipment needed to train the astronauts, building and maintaining the launchpad, paying the people on the ground in mission control, having rescue plans and personnel ready to get the astronauts or space tourists to safety if there’s an emergency. And that’s just the short list.

“It’s always a more complex answer,” Daniel Huot, a spokesperson for NASA, tells The Verge. “There’s not just a line item that says, ‘Send this person to space.’”

Here’s what we do know: while Musk wouldn’t specify an exact price, he did say that the around-the-Moon mission could cost roughly the same or a bit more than a crewed trip to the International Space Station. SpaceX declined an emailed request for clarification.

So what does that mean? Right now NASA pays the Russian space agency Roscosmos about $81 million and change for a round-trip ticket in a Soyuz capsule. The latest five seats NASA bought in bulk were a little cheaper, about $74.7 million per seat. Another spokesperson for NASA, Kathryn Hambleton, told The Verge in an email that the ticket price includes:

Training to operate the spacecraft

Use of the launchpad and launch support services

Flight control, docking, and undocking services

Air, consumables, and life support en route

Standby emergency services for a rescue in orbit, or during landing

Medical services after landing

The variable weight of the crew and their cargo to and from the station

The reason why NASA relies on Roscosmos is that the US space agency hasn’t had a vehicle of its own to ferry astronauts to and from the ISS since 2011, when the Space Shuttle program was shuttered. NASA is hoping the $81 million price tag will drop in the future by partnering with private companies like SpaceX and Boeing through the Commercial Crew Program.

NASA estimates that a round-trip ticket to the ISS on the SpaceX Crew Dragon or the Boeing CST-100 Starliner would cost about $58 million. A spokesperson for Boeing could not confirm the ticket price, and SpaceX declined to comment.

How much would this longer trip cost?

A one-way trip to the ISS, however, covers a distance of roughly 220 miles. Musk said yesterday that the SpaceX lunar trip would brush past the surface of the Moon and venture deeper into space, before looping back to Earth — a distance of approximately 300,000 to 400,000 miles. (It’s not clear how they arrived at those numbers, considering that a one-way trip to the Moon when it’s closest to Earth is about 225,623 miles, according to NASA.)

How much would this much longer trip cost? Space Adventures, a travel agency that arranges space journeys with Roscosmos for private citizens, tells The Verge the price tag is more than double the cost of a trip to the ISS: about $175 million dollars per seat. The company, which has already sent seven individuals to the space station, plans to send tourists around the Moon by 2020 — and that’s how much they’re going to charge.

“We cannot be more specific as there are many variables, including destination, vehicle, duration and other options,” Stacey Tearne, a spokesperson for Space Adventures, wrote in an email to The Verge.

One of the variables, which could shoot up the price of any space mission, is of course, delays. And as we know, Musk has a bit of a problem with deadlines.

Update 1:10PM EST, 2/28: Updated to reflect the fact that a SpaceX spokesperson replied to emailed inquiries after the story was published, but declined to comment.