NASA Administrator Jared Isaacman has pushed back on cost estimates for a proposed lunar rover built by combining two Mars rover engineering testbeds, warning that "we simply will not fly it" if the projected billion-dollar price tag proves accurate. The dispute highlights the challenge of NASA's push to find cheaper ways to explore the Moon, as the agency weighs whether repurposing hardware originally built as ground-based test units is really the money-saver it initially appeared to be.

The Planetary Society's Casey Dreier estimated the proposed PROMISE mission, which would merge parts of the OPTIMISM (Perseverance) and MAGGIE (Curiosity) testbeds into a single lunar rover, could cost between $723 million and $1.33 billion, including launch and landing costs of $234 million to $320 million. Isaacman did not dispute Dreier's breakdown but excluded launch and landing costs as unavoidable for any mission, saying "if PROMISE costs even 20 percent of your low-end estimate, we simply will not fly it." Turning the Earth-based test units into flight-ready lunar hardware would require qualifying components for launch, adapting them to lunar conditions, and reworking power and communications systems, including adapting a spare plutonium power source.