My wife owns and operates a private medical practice (she's an audiologist), and it's
insane the way she has to do billing and accounts receivable.
When a patient asks about prices she quotes them her standard price for the necessary services and calls their insurance company to confirm that the patient is covered and check what kind of copay the patient needs to pay. Assuming the patient is satisfied with the results of that call she provides the services, bills the patient for their copay, and files a claim with their insurance company.
Several months later (and no real way to predict when), the insurance company will provide an "offer" for probably somewhere around 60% of the quoted price (though this varies dramatically as well). She can accept the offer, in which case she's inevitably required by the terms to eat the difference instead of attempting to collect the balance from the patient. Or she can reject it and attempt to collect from the patient, in which case they will likely be very confused and angry as to why their insurance isn't being accepted, despite her explicitly confirming that it would be covered before they purchased her services.
To be clear, none of this is negotiated or agreed upon in any meaningful sense. She doesn't have any special relationship with any insurance provider. Instead, the providers use their massive power differential to dictate terms. She wouldn't be able to stay in business if she didn't accept at least some insurance, but they'd be quite happy to never write her another check.
She knows what her price schedule for everything is, but it doesn't end up mattering that much since she doesn't know when or how much she'll ultimately be reimbursed except in an extremely broad aggregate sense. The worst part is that she has to dramatically overprice her services so that she can ultimately get adequately reimbursed to keep the lights on, which just ends up hurting the patients who don't have insurance.