Good point! Having been on well water for a couple years I forgot about the costs of being on metered water. Amount of water wasted is dependent on the membrane, but also heavily impacted by water pressure and water quality. Sometimes you can boost the efficiency of an RO unit with an extra pressurizing unit (I have to use one of those now due to low water pressure out of the tap that was causing a ridiculous fill time and waste ratio).If the water supply is metered, it could cost quite a bit making your own RO as they do waste a lot of water. But if your water costs are fixed (eg based on the rateable value) it won't cost any more to make your own - apart from buying the equipment.