The cost isn't just in dollars though. In a PET scan they inject a radioactive dye into your bloodstream. It "exposes you to around the same amount of radiation that you would receive from the general environment over about three years." [1]
The high dollar cost may also be related to newer, more sensitive machines that don't require as much radiation exposure to the patient.
And it's not just any old radioactive source, it's a positron emission!
I believe the way it works is that the injected source emits positrons as it decays (having swiped them from the vacuum), shortly thereafter, these annihilate with the electrons of your body, and the scanner looks for the signature pair of gamma rays thus created, using the relative time of arrival to determine their common origin in 3-space. That reduces the problem of scanning to arranging for the decaying source to accumulate in the place we wish to scan which apparently can be done.
That we scan ourselves by annihilating antimatter with our bodies in targeted ways just seems so awesome to me.
The high dollar cost may also be related to newer, more sensitive machines that don't require as much radiation exposure to the patient.
[1] https://www.betterhealth.vic.gov.au/health/conditionsandtrea...