Reverse osmosis systems are generally more expensive than other water filtration systems. But they also offer the very best level of filtration and produce the purest water. The initial outlay can range from as little as $200 for a basic domestic unit, up to thousands of dollars for a whole-house system. Costs to consider when choosing a reverse osmosis system include: Installation - some systems are beyond the scope of DIY and require professional installation. Maintenance - more sophisticated systems may require professional maintenance. Filters - many systems use multiple filters that need changing anywhere from every one to every three years. Water use - reverse osmosis systems produce a lot of wastewater, so your water bills may rise as a result. However, this is far less noticeable with smaller, point-of-use units than it is with whole-house systems.