I use Gumbs Car Rental on St Barth. You can find pricing at http://gumbs-car-rental.com They have a booth at the airport, or will deliver a car to the ferry terminal. My advice is to do as d_fish suggests, and fly St Barth Commuter out of the Grand Case Airport. Lunch can vary from $30 for cheeseburgers, fries, and beers for two at Le Select to $400 - $500 for two after pillaging the wine list at a 5-star beachfront hotel.