It's cheaper to buy, for sure. But we don't know the costs. It's possible that these launches are far less expensive than SLS, but equally unsustainable.
How might that be? SpaceX doesn't have to manufacture whole new rockets for every Falcon Heavy launch, but even if they did, as far back as 2016 they could build 16 new cores (5 Falcon Heavy's worth) per year.
https://spacenews.com/spacex-seeks-to-accelerate-falcon-9-pr...