A barebones RO system is just that...an RO system with no additional, or maybe a sediment or carbon filter.
The RO is a membrane, which water is passed, is a very fine, porous filter which only allows the water molecules to pass. Everything else is filtered.
Each stage in an RO system increases the cost as well as contaminates filtered. RO doesn't filter contaminants such as herbicides and related man made compounds. An addition stage (Carbon) will. Right on down the line...through sediment filter, carbon block filters (sometimes 2) RO membrane, and on through a di-ionization cartridge and possibly even a final stage of a UV sterilizer.
All a matter of what you do not want in your water, and are willing to spend.
As mentioned prior, shop around. RO units made by aquarium orientated companies are usually far more expensive then domestic drinking water orietated companies.