A rental car will be cheapest. You pay for a 24 hour rental, not a calendar day. You'd be able to make a store stop, go out to dinner, or whatever. You can make a choice to keep the car (drop it on the way to the terminal and take car rental shuttle to the port), drop it off that night, etc...a whole new set of decisions.
Rental discounts are available at rentalcodes.com and mousesavers.com. I've paid $46 and $92 for a mini-van, typically under $50 for a full sized car including all taxes and fees.