In Arthur v. Catour, No. 3-02-0810 (3d Dist. 2004), the Third District considered this issue: "whether an injured plaintiff may recover as damages the entire amount billed for medical services, or if she is limited to the discounted amount paid by her insurance carrier."
The issue was important enough that an amicus brief was filed by the Illinois Association of Defense Trial Counsel, which argued the plaintiff would receive a windfall if she were allowed to recover more than was actually paid by her insurance carrier. The Third District disagreed (Justice Holdridge dissenting), applying the collateral source rule, which states benefits received from a collateral source do not diminish a plaintiff's damages.
The result: The plaintiff was allowed to recover damages for the amount her health care provider billed, not the lesser amount her insurer paid as a result of its separate negotiations with the medical provider.