We rented a van for 6 people at Maui through NCL. It cost $89 for 12 hours. We were picked up at the pier and taken to the rental agency. You pay for the rental and then you fill up the gas tank before you turn it in. We did the Road to Hana and had to put $28 worth of gas in the van. It ended up costing us a little over $20 each for a whole days worth for driving.
Another couple just got in a shuttle at the pier and rented a car and their car was just $29 for the day. They used about the same amount of gas. Gas is running $3.30 a gallon.
There were shuttles from every car rental agency that you can think of at the pier, so it would be no problem getting to the rental places. The shuttles bring you back to the ship after you turn the car in. You have to check to when the last shuttle runs to the ship, and just be back in time.
The next day 7 of us took a Taxi van to Lahaina. It costs about $75 each way ($12) each person. We took the Taxi drivers phone number and called him and he came back to get us in the middle of town. That way we knew how much we would have to pay to get back to the ship. Lahaina is a beautiful little town with lots of shops. It was one of our favorite places to go.