Prepare yourself for some wild numbers!
Check this out San Diego! You want to see a crazy cost comparison? Assume that you have a 2000 square foot house. Not crazy big for SD. At 600 gallons/1000 square foot/inch of rain, this looks like 12000 gallons of rainwater coming off your roof for the year. Assume you wanted to store most of it, and you have room for 9000 gallons of storage on your property (above ground, round plastic tanks are cheapest). You may want all the water to keep your 500 square foot lawn green (which wouldn't quite be enough anyway) or maybe you have a thriving food forest which is providing you with high quality organic food (that sounds delicious!)
If you add up all the $$ it's going to cost to install 9000 plus gallons of rainwater storage, it will be in excess of $10,000. If you want to tie it into a pump and feed it into the irrigation system that will add another $2000 or so.
That comes out to over $1/gallon.
Now let's look at shower greywater for a four person household:
Mom showers 15 minutes a day (are they using low flow showerheads (2gallons/minute)?) = 30 gallons/day
Dad showers 5 minutes a day = 10 gallons/day
2 kids at 20 minutes a day = 80 gallons/day
Total = 120 gallons/day x 365 gallons for the year = 44,000 gallons +/- for the year
If you are installing a permitted system for 2 showers to gravity flow into mulch basins on your property, this could add up to maybe $5K, on the high side. Note, you don't want to use this on your lawn!
That comes out to $0.11/gallon
Now if you want to compare that with water coming out of your tap. If you look at your water bill, you are charged per HCF (1 Hundred Cubic Feet = 748). Depending on which tier you are in and what city you live in this could be about $3-6/1HCF which adds up to about to between $0.0004 and $0.0008 per gallon.
Water is cheap San Diego. But everything comes at a cost! There's tons of literature about the cost of bringing water to San Diego from hundreds of miles away from the Colorado River and from the Delta up in Northern California. Make an informed decision.