Depends on what you are allocated to start off with, and whether you do it before you go or when you get there. There are as many people who get good deals on car hire as there are those that get ripped off.
People carriers are amongst the most expensive type of car to rent, one way of finding out the cost is to find out the car provider and look at their website and look at the difference in price between what you are allocated and what you want. This will bascially be what it should cost you. Or look at someone like Airline Network or Us Rent a Car for a comparison.
You could take a chance on getting a cheaper deal when you arrive, but down to whether you want to take the chance.