One would need to take quite ...

One would need to take quite a few aspects into consideration 

1. Capacity 

2 Capex 

3 O$M Cost 

Many existing plants have already been constructed world wide so collecting data based on their construction cost and processes should be readily available  , if not get in touch with a civil engineering consultant Firm like GIBB or Royal Haskoning who specialize in these projects globally.

The most important factor to consider is the O$M cost because that will be the deciding factor , will the plant generate enough money to sustain itself over the life period , is the community willing to pay for water services can they afford the services are national frameworks in place , is there land available  for the project how many people will have access to water generated from the plant.

The capital cost will be determined by the capacity of the plant , have you got trained people and qualified people to run the plant , what type of water are you treating river water or ground water each plant has different types of treatment mechanisms 

my advise is to contact the local civil engineering consulting firms and get all info from them ,