https://arstechnica.com/science/2019/11/the-white-house-puts-a-price-on-the-sls-rocket-and-its-a-lot/After the Senate Appropriations Committee released its fiscal year 2020 budget bill in September, the White House Office of Management and Budget responded with a letter to share some "additional views" on the process. This letter (see a copy), dated October 23 and signed by acting director of the White House budget office Russell Vought, provides some insight into NASA's large Space Launch System rocket.
Congress has mandated that NASA use the more costly SLS booster to launch the ambitious Europa Clipper mission to Jupiter in the early 2020s, while the White House prefers the agency to fly on a much-less-expensive commercial rocket. In a section discussing the Clipper mission, Vought's letter includes a cost estimate to build and fly a single SLS rocket in a given year—more than $2 billion—which NASA has not previously specified.
"The Europa mission could be launched by a commercial rocket," Vought wrote to the chairman of the Senate Appropriations Committee, Alabama Republican Richard Shelby. "At an estimated cost of over $2 billion per launch for the SLS once development is complete, the use of a commercial launch vehicle would provide over $1.5 billion in cost savings. The Administration urges the Congress to provide NASA the flexibility called for by the NASA Inspector General."
Independent estimates have pegged the SLS cost this high, but NASA has never admitted it. A $2 billion cost to launch one SLS rocket a year raises significant questions about the sustainability of such an exploration program—the government killed the similarly sized Saturn V rocket in the early 1970s because of unsustainable costs.