Never really figured it out. Mine is a 1:1 system (makes a gal wastes a gal). Lets see. I go thru about 5 tanks a month some times 6. So figure 60x5. 300 gal a month over 24 months. So 7200gal every 2 years. Used water 14400gal. So .01 per gallon plus county water charges. 1.34 per 1000 gal so...