Starship has not yet flown even a fraction of what SLS has, so I think the comparison is premature. If it takes another ten years to get to a point that it can successfully achieve its Artemis objectives, I doubt it will remain cheaper than SLS. And given that it has already been delayed way beyond the first estimates for when it might be ready (it was supposed to have flown to Mars with astronauts on board by 2022, I believe), I don't see why another 10 years is any worse an estimate than others.

SLS has flown once. What are you talking about?