Here in southern Ohio, various air evac outfits are used according to availability. When we had our motorcycle wreck in 2012, the first called flight had trouble. EMS transported my wife to the local hospital landing pad and she was evac'd on another flight to the Cincinnati Level 1 trauma center. I've seen the same issue in other situations where it takes 2 or 3 flights before they get one actually to the scene.
In our case, they initially billed $16K (60 mile flight). Medicare paid around $2K and they accepted that, we were billed an additional $500. Those limits are set by Medicare. If they accept Medicare, they cannot charge more than the assigned amount. (Local EMS also charged an additional $1000. Geesh, I used to do it at no charge!)
Here locally, EMS will put a flight on standby or in the air even before they arrive on the scene. The fire department is dispatched to set up a landing zone. OTOH, just across the river in Kentucky, the EMS always transports to the local hospital landing zone and meets the copter there.
Bottom line, I would be very cautious about signing up with any particular company since they may not make the flight. Check your health insurance to see what is covered. If you feel the need for more coverage, look for it as part of general coverage, not with a particular company.)
On a related topic, I remember reading a story where, in NE Minnesota, the air evac flight has to land and refuel before they get to the pickup point! Ouch!