The maintenance cost of a good internal domestic installation should be quite low especially now with the new LED lighting available. However, maintenance of an exterior installation, garden lighting for example, is ongoing.
Without doubt, no matter how good a garden lighting system, there will be problems with water ingress somewhere along the line. Garden systems should always be installed with easy access to all junctions and terminations, allowing for quick efficient repairs when a fault occurs. Please be wary of this if you are designing a lighting project for your garden.