A basic insurance is always included, but the amount of the excess (what you will have to pay if you have an accident) varies depending on the rental operator. This amount is always stated on the vehicle listing page. 

You can usually choose to reduce or even cancel this excess for a daily fee that will be at your charge (also called CDW option), which means that in the case of an accident, you will not have to pay anything. This extended cover option can usually cost from $10 to $40 per day depending on the rental operator.