Seems pretty easy to quantify - it looks like there are ~37K motor vehicle deaths in a given year [0] and "Approximately 6.2 million patient transport ambulance trips occur annually." [1]

Ambulances would need to save their passengers' lives <1% of the time for them to "break even" so to speak.

[0] https://en.wikipedia.org/wiki/List_of_motor_vehicle_deaths_i...

[1] https://www.nhtsa.gov/staticfiles/nti/pdf/811677.pdf