I do not believe there is a word that will fit your sentence as it is structured, and be clearly understandable.
However, the idea you are talking about (sharing the cost of a potential risk), is a form of insurance
Insurance is a means of protection from financial loss. It is a form of risk management primarily used to hedge against the risk of a contingent, uncertain loss.
With their payments being insurance premiums.
Premium is an amount paid periodically to the insurer by the insured for covering his risk.
That is, your friends are paying a small insurance premium, to cover the cost of your car against the potential risk of damage.
This said, insurance will tend to suggest that you are paying a small amount to a large company - on the basis that they cover the entire cost of repair if something does go wrong. I don't feel you can communicate the exact arrangement clearly - without including the details of it:
Each passenger paid an insurance premium of $10 to cover the potential risks to the car.