NASA does not deny the “over $2 billion” cost of a single SLS launch.

For the first time, a government cost estimate of building and flying a single Space Launch System rocket in a given year has been released. This estimate of "over $2 billion" came in the form of a letter from the White House to the Senate Appropriations Committee first reported by Ars this week.
In the nearly decade-long development of the SLS rocket, NASA officials have studiously avoided providing a so-called "production and operations" cost. This is partly because it can be difficult to estimate flight costs during development, but also very likely because doing so might give lawmakers who have backed the project some measure of sticker shock. After all, a fully expendable version of the Falcon Heavy rocket, which has two-thirds of the lift capacity of a Block 1 version of the SLS rocket, can be bought today for $150 million.

After publication of the White House letter, the agency did not deny the estimate that producing and flying one SLS rocket in a given year—which is the production capacity core stage contractor Boeing may be able to reach by the early 2020s—will be more than $2 billion. The first SLS launch could come in 2021.
"NASA is working to bring down the cost of a single SLS launch in a given year as the agency continues negotiations with Boeing on the long-term production contract and efforts to finalize contracts and costs for other elements of the rocket," an agency spokesperson, Kathryn Hambleton, told Ars.

Don’t forget development costs

The White House number appears to include both the "marginal" cost of building a single SLS rocket as well as the "fixed" costs of maintaining a standing army of thousands of employees and hundreds of suppliers across the country. Building a second SLS rocket each year would make the per-unit cost "significantly less," Hambleton said.
What the White House cost estimate did not include, however, was development costs. Since 2011, Congress has appropriated approximately $2 billion per year for the "development" of the SLS rocket (this does not include hundreds of millions of dollars spent annually on ground systems "development" for the rocket at Kennedy Space Center). If these costs are amortized over 10 launches of the SLS vehicle during the 2020, the per-flight cost would be approximately $4 billion per flight.
Moreover, this is just for the SLS rocket's core stage, side-mounted boosters, and a basic upper stage. Developing and adding the Exploration Upper Stage will add hundreds of millions of more dollars. Then there is the cost of the Orion spacecraft, which at approximately $750 NASA recently valued at approximately $750 million for the first six missions. million for the first six missions.
Adding all of this up, the true cost of a Space Launch System mission with Orion on top in the 2020s, including the rocket's development but excluding ground systems and Orion development costs, appears to be in the ballpark of $5 billion per flight. Let's hope the astronauts are served more than just pretzels after takeoff.