If your not dealing with a salt intruded well or living on a private island then it is usually over kill.
Calcium
Iron
Tannin (yellow color)
Sulfur
All of these can be dealt with with simple water filters if you use the right ones and set them up properly (And all but tannin can be dealt with without a chemical additive…they just use air as an oxidizer)
An R.O would usual only be for drinking or Growing (high dollar) crops.
that being said it comes down to how much you need (per day / at one time)
i could run a whoole house on a few inexpensive filters and use a large holding tank and keep 3 Teenage Girls supplied with Unlimited Showers (not an easy task) or i could use a huge set of filters and a smaller tank for space savings at the cost of $$$$
Mine at home cost about $350 Retail and makes 100 GPD
i can add 100GPD over and over for about $80 per R.O. Membrane + the water flow i need to keep up (usually not an issue until we start getting in to more than a few membranes)