A few weeks ago we got to the island about 11:00 and decided to rent a cart around 1:30-2:00. Had no problem getting one for the four of us for $32USD for 2 hours, which was plenty of time to go around the island and stop for quite a few pictures.
in April 2009 I paid 42 dollars US for the entire day. But it seems to me we had to make a deposit also.