WASHINGTON — Since a U.S. Air Force estimate emerged in September, putting the cost of President Donald Trump’s desired Space Force at $13 billion, Pentagon officials have been pledging that the “official” cost estimate from the department will be much smaller.

Now we know by how much.

Speaking to reporters Thursday, Deputy Secretary of Defense Patrick Shanahan said his team’s initial estimate for the Space Force will be in the “single digits” of billions of dollars, and “could be” lower than $5 billion.

The difference in cost is significant, not just for the dollar value but as part of the broader fight over the future of the Pentagon’s space architecture.

The $13 billion figure sent waves of sticker shock through the defense community and led to accusations that the Air Force — which has been reluctant to embrace the idea of a Space Force — was hyping up costs to kill the idea.

During the Defense One conference on Thursday, Air Force Secretary Heather Wilson was asked about Shanahan’s estimate and pointedly defended the figure put forward by the Air Force, saying that the $13 billion sum is needed to realize the scope of Trump’s direction to the Pentagon.

“Whatever is put forward needs to implement the president’s proposal. What we put forward was in the cost estimates to implement a standalone department," she said.

"The president is going to be making some decisions to put forward a proposal in concert with his fiscal year 20 budget proposal that will go to the Congress in February. So the cost will be really based on what are the elements in the model in that proposal, and our cost estimate that we gave to a lot of people in the Pentagon is September was the cost of a fully fledged standalone department and also a unified combatant command.”

In an exclusive interview with Defense News last month, Shanahan pledged that his cost would be “less” than the Air Force figure.

“The goal here is not to create a lot of incremental cost,” he had said. "In this department, you know with this secretary and this Congress, people in the White House, they’re not going to let us just go throw money at that.”