Tickets are approximately $2 per person and $20-$30 per vehicle (the price varies depending on the size of the vehicle). These can be purchased in advance, however you will need the license plate number in order to book. You can book them here: https://www.quickpaycr.com.
If you do not purchase the ticket before hand, then there is a ticket counter available for purchase at Musmani Bakery across from the ferry landing. There will be a ferry representative handing out pre-boarding car tickets for the line of cars-you need this ticket to buy the actual car ticket (this is not needed if you purchase in advance as you just go to the front of the line and the staff will have your ferry ticket printed for you (you do need to show proof of purchase which will be emailed to you).