The Falcon Heavy can ship can ship 16.8k kg to Mars [1] and that's at a higher delta V distance/cost than TLI. In general the SLS can carry around 50% more than a Falcon Heavy, but the Falcon Heavy (fully expended) costs 5-10% as much.
And yes, the cost difference is absolutely staggering.
It's unprecedented. A large part of Startship's technical risk in the Artemis programme is in-orbit refuelling.
That said, if IFT-5 or 6 can demonstrated recapture and an in-orbit fuelling attempt is made in 2025 that doesn't end catastrophically, I think the centre of mass of technical risk for Artemis shifts away from the novel fucking launch vehicle system that SpaceX has been tasked with building from the ground up to checks notes a rocket based on refurbished Space Shuttle engines and double checks this glorified launch tower.
Starship will be a game changer in the most literal sense here as long as they figure out payload doors…