Hospital's costs don't magically change though. The MRI machine? Expensive as all hell. The liquid helium (or whatever) to run it? Also expensive as hell. Doctor's wages? Nurses wages (they're unionized, fwiw). X-ray machines, X-ray tech's, the cost of reagents, pharmaceuticals and all other medical supplies?

The hospital, a for-profit business in the United States, isn't voluntarily going to give up profit, so they "hack" the system by raising prices. If InsuranceCo says they'll payback 20% of list price, but it costs the hospital $100 to run the test (including some nominal profit; hospitals aren't non-profits, for better or worse), the hospital must charge $500 for the test, just to break even on InsuranceCo's patients.

Which they aren't refuse, because those patients are a significant portion of the hospitals revenues, or due to laws, in some jurisdictions. ER's can't (legally) turn away people they don't like (Eg African Americans or immigrants).

“If InsuranceCo says they'll payback 20% of list price, but it costs the hospital $100 to run the test (including some nominal profit; hospitals aren't non-profits, for better or worse), the hospital must charge $500 for the test, just to break even on InsuranceCo's patients.”

I have never seen a real accounting of these costs so we can see whether they overcompensate. It’s the same with the high drug prices. They always say that other countries underpay and therefore the US has to pay more but you never see any real numbers.

MRIs and most imaging is a huge profit center. Manufacturers will approach providers to form an "imaging group", promising recoupment within 18 months for the highest end machines.

And is been shown that doctors who belong to such are more than 2.8 times more likely to refer patients for imaging.