Why does an MRI cost so much?

A simple rule of capitalism is that if you have something that others want, they will pay you whatever you want to get it. The wise capitalist sets the price so that he gets the most return for the most reasonabe amount of work - not so high that he has to make all of his funds on one patient, not so low that he has to work like a dog to make ends meet.

In the case of MRIs the equipment is costly, the maintenance is expensive, the operators are paid well, and insurance to cover mistakes is prohibitive. As a consequence the supply is low. At the same time the demand is high and increasing.

A textbook example of supply and demand.

ASIDE: The fact that folks need the service and are worse off for not getting it does not enter into the capitalist system of evaluation.