Here’s an interesting report on the economics of residential graywater systems. It’s a little wishy-washy (no pun intended, ha) on the numbers, but it has some links and references that could be useful. From a quick skim, it suggests that if you can achieve a savings of about $200 a year, your system will break-even over a typical service life of around 15 years. This is more likely to happen if you have a relatively high number of people in your house and if you have relatively high water rates.
I have a low-tech, essentially free graywater setup. I turn on the shower, wash my hair and face with weird chemicals, stop up the drain, and wash the rest of myself with pure, non-toxic biodegradable soap. Then, I use a bucket to collect water for houseplants and outdoor plants. I check with NOAA online to see if there has been less than an inch of rain over the past 7 days (a very rough rule of thumb for evapotranspiration around here) and to see if there is rain expected over the next day or so. If I’m diligent about this in the summer, I end up not having to get out the hose too often. Combine all this with a rain barrel or two and I would have to get the hose out even less often.
If I were to accidentally pee in the shower…well, I’ll take the 5th on that one but I’m pretty sure the plants wouldn’t mind.