When I needed a similar non-emergency procedure (cervical, rather than lumbar, MRI), my neurosurgeon, at a large, university-affiliated hospital, referred me to the in-house radiology department.

Within a day or two, I received a phone call from some department at my health insurance provider telling me "Hey, we see that you are going to get an MRI; did you know that you can get the exact same procedure at providers A, B, and C, for a lot lower cost?" They did this even though the hospital's radiology department was in-network.

In this case, the cost difference was similar to what's noted here - about $4,000 vs $450. Since the insurance company was paying most of it, aside from my copay / share of cost, they were motivated to provide me a little transparency.

To be honest, as a patient, I would have preferred to get the MRI at the hospital; their facility was nicer, cleaner, more modern, and there was better data integration for getting the results to my neurosurgeon and keeping them as part of my holistic medical record. Nonetheless, I went to the cheaper provider.

Like any opaque market, bigger players will have more info, but I doubt anyone will share information as its collusion (maybe illegal) and also advantages either your negotiating opponent or your competitors...

Exactly. Small payers are actually pretty hungry for this data. Generally, big payers are more privy to market rates (by paying for datasets, owning clearinghouses, etc).