I always thought this was the case but I have to wonder.
recently I was catching up on filing our insurance paperwork (eob's we get that show what was billed/what allowed/what insurance paid/what max provider can bill us per their contract) and I noticed that with both dh and I once we got supplemental coverage on top of our existing health insurance some providers changed the amount (increased) for certain types of appointments. we did not get the supplemental at the same time as each other, so it's not a matter of it just being a coincidence that the month our coverage increased (2 pools of insurance money to bill from) the doctors decided to increase their fees, so the only thing I can think is once the doctor's billing service saw the secondary coverage they increased the fee for service to whomever rate they had the higher contract rate with
btw, in our region there's a search engine you can use to find out the average local cost range for different medical tests and procedures. just for the heck of it I just did a search on it for ct scans-they range from a low of $925 to a high of $2500. the lower charging providers (from my experience) are the independent imaging centers who do the highest number annualy. the highest charging-the hospitals. I found similar results when dh needed an MRI a few years ago and our PPO had no preferred providers in the area (so much bigger out of pocket cost for us). dh's doctor (preferred provider) recommended a place but I explained that we needed to minimize costs so he suggested we call around, which we did and the difference between providers was as much as $2000