The cost of a greywater system depends upon the size and scope. The most basic greywater systems involve only a bucket or basin and perhaps a garden hose while elaborate systems require water from sinks, bathtubs, showers, and clothes washers to be piped into a storage tank then pumped into an irrigation system.
The maximum average expected water savings from any greywater system is about 50,000 gallons per year for a family of 4. Assuming a relatively high water cost of $3.00 per thousand gallons, this amounts to a savings of about $150 per year. With a maximum savings of $150 per year, it is difficult to design a truly cost effective greywater system.
According to one of the leading greywater web sites, the huge majority of "successful" (i.e., cost effective) greywater systems are so simple as to not be noticed by regulators, manufacturers, consultants and salespeople.
Simple and basic greywater systems are far more likely to be cost effective than expensive packaged systems or even custom designed systems.