I'm not a doctor, nor am I close enough personal friends with one to ask about pricing. However, I did find this thread with optometrists talking about pricing and they don't seem to mention any difference in pricing between those insured and not.

I don't want to place motive on actions without evidence, but it is possible that the practitioners simply feel sorry for you if you have no insurance and discount as much as they can.

That said, I do know that there are negotiations between doctors and insurance companies as to how much money changes hands for insured patients. If you're going to apply free market principles to one side of the equation, you need to apply them to the other, too, and blame the insurance company for not being a better negotiator.