A piece of anecdata on what happens when it's not as opaque: I took the script for a lumbar MRI to a standalone imaging chain advertising low costs. The phone quote was 1/10th the price quoted by the hospital. What puzzled me was that although the imaging place accepts my insurance it asked for $395 if I wanted payment to apply against my insurance deductible but only $275 if I paid without going through the insurer. Was the delta reflecting the imager's cost to file a claim? They couldn't tell me. I wish there were ombudsmen to turn to.

The price differential is based on the cost of filing an insurance claim, the risk that the insurer will deny the claim (or require changes before approval), and the delay in receiving payment. A cash payment today is a sure thing.

Actually there's at least 2 incentives: 1) even with insurance coverage you have a deductible, and unless you have a year with a lot of doctor visits or procedures, you're probably going to end up paying almost everything out of pocket, 2) higher medical bills drive insurance costs, if you can get lower bills you can reduce your insurance costs (not even negotiating with the insurance, but just simply switching to a different/cheaper plan with lower coverage).