I'm not comparing the bill uninsured patients pay to the bill insured patients pay. I'm comparing the bill hospitals charge uninsured patients to the bill hospitals charge insurance companies for insured patients.

Hospitals and clinics offer a preferential rate to insurance companies as a sort of volume discount because insurance companies with thousands of patients have a lot more negotiating power than any individual patient who is uninsured or using an HSA.

It got even worse once health insurance companies started negotiating contracts with favored nations clauses requiring hospitals to bill then ~10-30% less than patients paying out of pocket. It's an oligopoly, but on the demand side: an oligopsony.