Last time I checked their catalog, Water Treatment Facilities R Us didn't have a prepackaged price...
My guess would be the cost of a water treatment facility is driven greatly by the number of users it will service. It will also be driven by the quality of the water it will be treating and the quality of the water it will be dispensing. Another cost-bearing consideration will be the facility's proximity (topography and geograpy) to the end users and the pumps and capacity required to service the end users.
Considering all these variables, I don't think it would be easy to establish an "average" cost per facility.