According to Home Water Works, the average American shower lasts 8.2 minutes at a flow rate of 2.1 gpm, leading to a total of 17.2 gallons per shower. ( http://www.home-water-works.or... ) Based on 4 showers per day per household, that's 68.8 gallons per day, meaning $400 would buy you almost exactly 10 years worth of showers at the average flow rate. Assuming the maximum savings of 70%, you're reducing your daily usage from 68.8 gallons to 20.64 for a savings of 48.16 gallons. This means you're looking at over 14 years before you break even. Best case scenario.