The answer is health plans according to a new paper by Sen, Chang and Hargraves (2023):
Using the Health Care Cost Institute’s data set of claims for one-third of the US population with employer-sponsored insurance, we found that unadjusted prices were higher in self-insured plans for most of the services we studied, with the largest differences found for endoscopies (approximately 8 percent higher in self-insured plans), colonoscopies (approximately 7 percent), laboratory tests (approximately 5 percent), and moderate-severity emergency department visits (4 percent). When patient characteristics, plan type, and geography were adjusted for, differences were generally smaller but were consistent with these findings. Higher prices in self-insured plans suggest that there may be opportunities for employers to lower prices and for policy makers to act where employers have limited leverage to negotiate with providers.
Read the full paper here.