Full day trips are approx. Like most things in life, supply and demand drives price. Kona has a hundred boats in a world-famous Marlin fishery. Competition is tough, and there are some extremely dishonest operators in the harbor who will steal charters by offering lower prices. Demand simply isn't high enough to allow all these boats to charge premium rates, and thus, their average price is the lowest in the state.
Demand is high and supply is low.
Also, Lahaina is a good 30-45 minute run to the fishing grounds, so they run harder, which burns more fuel and adds to the maintenance. Oahu boats, while close to the fishery, are cursed with a tough location to attract foot traffic. People have to want to fish by pre-planning a trip before leaving home, or go through the phone book in their hotel room when they arrive. Parking is tough, and the state simply hasn't done much to help support the fleet by way of harbor improvements or proximity to the customers. Given these constraints, Oahu boats do surprisingly well, and the boats we refer will give a great trip at a very reasonable price.