"We found that the first four Artemis missions will each cost $4.1 billion per launch, a price tag that strikes us as unsustainable," NASA Inspector General Paul Martin said during a meeting of the House Subcommittee on Space and Aeronautics.

Martin said the $4.1 billion estimate is only for production costs and ground operations, "and does not include development costs required to get the Artemis program to this point in time.".

"The core national transportation elements of the Artemis programs have been developed for a fraction of the cost of the comparable Apollo program systems and will continue to become more affordable as the programs transition from development to operations," Boeing added.

Beyond a development cost at a fraction of SLS, SpaceX also expects the cost per launch will be far less expensive, with Musk saying last month that he is "highly confident it would be less than $10 million.".

Starship is also important to NASA's Artemis program, as SpaceX last year won a $2.9 billion contract to develop a moon-specific version of the rocket to serve as the crew lunar lander.