I don't think "no idea" is fair. We don't have exact numbers, but there are various statements out there that give clues. Even the highest estimates I can put together put Starship far cheaper than SLS.
replyYou have to consider that Starship has not reached anywhere near the operational goals for Artemis, and there is no realistic time line for when it might. So we really do have no idea how much it might cost by the time it reaches the milestone SLS has already cleared (successful flight in lunar orbit, with a full payload that it successfully deploys).
replyYou also have to consider that SpaceX has the fastest, most reliable,
most cost efficient launch service in operation ever, and are using the same methodology to develop the most advanced launch system ever attempted.
replyEven if a Starship needs to be scrapped after landing, the Super Heavy booster works, returns nominally to the launch site, and can be reused. This alone should make the whole thing cheaper than SLS.
replyOnly if the SuperHeavy booster can achieve the same performance as the SLS (payload to orbit), with similar levels of operational complexity.
replyThe SLS has already proven it can fly to lunar orbit and back on one single launch. In contrast, even if everything goes according to plan, Starship requires at least a dozen re-fueling flights while it hangs in orbit around the Earth to be able to then fly to the Moon.
Will one Starship launch, when it eventually works, be cheaper than SLS? Very likely. Will 12+ Starship launches + the time in orbit be cheaper than a single SLS launch? Much, much less likely.
Actually, we already know that with booster reuse disposing of 12 tanker starships will cost less than an SLS launch and actually be able to get to the moon, which SLS with Orion can’t actually do.
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