GAO: True Cost of SLS, Orion Unclear, Space News
"NASA has not released comprehensive, long-term cost estimates for SLS and Orion. The reason is to avoid giving Congress sticker shock, said William Gerstenmaier, NASA's associate administrator for human exploration and operations. "If we laid out a path directly to Mars and we laid out all the vehicles and all the testing and all the work to get there, then you end up with a fairly long period of time with a lot of funding that goes into that activity that says this program is something maybe we don't want to go do," Gerstenmaier said in November during a panel discussion with SLS and Orion prime contractors at the Newseum in Washington."
GAO Sees Through NASA's SLS/Orion Smoke and Mirrors, earlier post