A methodology is presented to assess the life-cycle costs of combined or hybrid floating offshore renewable energy systems, which is also applicable to general floating offshore renewable energy devices. It accounts explicitly for their life-cycle stages: concept definition, design and development, manufacturing, installation, exploitation and dismantling. It is a tool for decision-making and strategic planning, enabling a better understanding of the technological solutions and the aspects that might accelerate or decelerate the development of the industry. The method presented can be useful for comparing different types of floating offshore hybrid renewable energy technologies in terms of costs. The general methodology has been applied for the particular case of two specific hybrid systems: the W2Power and the Poseidon. Results for two locations in Portugal (São Pedro de Moel and Aguçadoura) indicate that the exploitation, manufacturing and the installation costs are the most important ones, with the exploitation cost being the most important for the W2Power and the manufacturing cost being the most important for the Poseidon. The Levelized Cost Of Energy is lower in São Pedro de Moel but when considering the technology the W2Power has lower Levelized Cost Of Energy in the single unit scenarios and the Poseidon in the farm scenarios.