IF we are dealing with a ...

IF we are dealing with a lower pressure boiler system where steam quality with respect to silica is irrelevant, then ion exchange (sodium-cycle) to soften the water is sufficient, and far less expensive.  Slightly more expensive in terms of capital, membrane capacitive deionization can be tuned to the precise purity required, with the exception of no silica (or probably boron) removal.  Membrane capacitive deionization has the lowest specific energy input of any desalination technique, thus operating costs are expected to be low.

Reverse osmosis is only needed when there are severe restrictions on the quality and quantity of boiler blow-down, or there is a definite need to remove most/all of the silica (boron, also).  Systems in this range of treatment then become expensive to invest in and to operate, but it is a necessary evil.  Usually at such a level, the RO may be multi-pass, may involve feed recycling, and can and perhaps be followed by downstream ion exchange, and/or Continuous electro-deionization equipment for final purity required.  Such equipment nearly rivals the output quality and quantity of that employed in semiconductor manufacturing.