Natural Filtration
Ultra filtration/RO alone will not remove all of the lighter, low molecular weight volatile organics such as THM's, TCE, vinyl chloride, carbon tetrachloride, etc. They are too small to be removed by the straining action of the RO membrane. Their chemical structure is such that they aren't repelled by the membrane surface. Since these are some of the most toxic of the contaminants found in tap water, it is very important that a well designed carbon filter be used in conjunction with the membrane.
In some applications, AC is used before the membrane. In ALL applications with quality RO systems, there is AC after the membrane. This means that post AC filters don't have to contend with bacteria and all of the other materials which cause fouling and impair performance if AC follows a well maintained membrane. Not all RO systems are created equally. That is why you'll see such a variation in price. The engineering and experience behind the RO design is crucial to it's overall performance and dependability.