With Airbnb, the compensation is variable and is set based on the owner's perceived value (which factors in the 'emotional piece of mind’ too among other things). It’s essentially a market based approach, and to get a wider acceptance from the supply-side, that’s what FlightCar needs to incorporate while preserving simplicity.
As an optimist, I believe the creases will get ironed out eventually. It’s not going to be easy (never is) and it might take some time, but it will happen. Here’s a structure that comes to my mind:
Tier 1 – Free parking and $10 for every day the car is rented out (like they have now)
Tier 2 – Pay $X per day to park and Receive $Y for every day the car is rented out, where X is the market rate which in SFO’s case is $18 and Y is the daily rental-price you name.