For medical emergencies in the Canyon, the Park will provide helicopter evacuation to the South Rim, and basic treatment at the clinic there. Insofar as I know, their past practice has been to not charge for that service.
However, if a sick/injured party has to be transported to Flagstaff or elsewhere for advanced care, those costs will be charged to the patient. And if that transport is by helicopter, it will be a very large number...
The good news is that many standard health insurance policies cover the latter circumstance. But not everyone has a policy with that provision. So on trips I've organized where someone did not have that coverage, I've recommended they look at this company.
Travel Guard - Medical Evacuation Insurance MedEvac Plan
Hope this helps.