“Osmotic power or salinity gradient power is the energy retrieved from the difference in the salt concentration between seawater and river water. Two practical methods for this are Reverse electrodialysis (RED), and Pressure retarded osmosis (PRO)” according to Wikipedia.
Not sure how reliable the source is but its presentation title is promising “Capital and O&M Costs for Membrane Treatment Facilities Capital and O&M Costs for Membrane Treatment Facilities”.
A more political and technical paper (2006) gives a good overview and equally breaks down the costs. It is certainly worthwhile comparing the cost structure as-was in 2006 to today’s structure. This should help assessing where the biggest gains have been achieved and what the possibly path to future success may be.