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.
You 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).
You 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.
We also have to consider the other major Musk lead company Telsa had the best selling car in the world and string of successful cars leading up to that before completely shitting the bed on the Cyber Truck.
I want Starship to be a success and reduce the cost to orbit and beyond, but past success does not in any way guarantee future success.
