Every transportation system has three components: rights-of-way, termini, and vehicles.  For the auto transportation system public funding sources typically only pay for the maintenance of the rights-of-way (roads). Termini costs (parking) are usually paid for by developers who are required to meet min off-street parking standards and then lump the costs in with the price of rent, goods, or services. Vehicle costs are then borne by individuals who buy personal autos.  The cost of owning and operating the average car in Hawaii is about $9,000 per year, though this excludes most of the costs of parking which are bundled into other costs. In total, Oahu residents spend $6-8b/yr on their cars.
Transit, on the other hand, always seems very expensive because cities have to pay for all three components upfront: guideways, stations, and trains. Yes, the rail project is expensive, but when you consider what Hawaii residents spend on transportation overall, the costs are not that big.