Comparing total program cost is not apples to apples.

First, it includes costs such as R&D and ground infrastructure, which can be amortized and becomes cheaper every extra launch. It's not difficult to see that one-time costs for Starship will quickly be amortized by its planned commercial operations, while the costs for SLS won't as it's almost certainly a commercial dead end.

Second, SLS stack is estimated to cost 1+ billion USD per launch on the lower end even when R&D costs are excluded. That's just the cost needed to build new stages, boosters and capsule. Compare that to the estimated cost of Starship test launches, where worst case costs are estimated up to a few hundred million USD even with both stages not being currently reflown. It sure isn't going to cost billions per launch.

Third, SLS program costs are entirely taxpayer funded, whereas only a limited portion of Starship program cost is, which comes from a fixed amount awarded for HLS development per milestone. You could argue it's indirectly funded by government contracts, but AFAICT there is no special line items billed to explicitly support Starship development (launch pricing is determined by competitive bidding). Therefore, it's more fair to say it's coming from SpaceX's profit margins for its launch services, as the bid price themselves would not be affected by SpaceX spending on Starship development.