Originally Posted by KarenM
Plain RO water costs me about .60 per gallon (in Austin, TX) from a local LFS. I have 3 gal bottles because they're easier for me to handle. I usually buy about 6 gal of premixed saltwater from that same LFS for water changes every 2 weeks ($1 per gal), and about 15 gallons of plain RO water to top off between changes. I have a 40 gal with a fan on it, so it evaporates about 1 gal per day. It all adds up to around $30 per month. I'm going to take everyone's advice and buy my own RO unit because it's just more convenient.
At $30/month, the home RO/DI unit will have a Return on Investment in about 7 months, which included any future membrane changes. Of course this is unit cost and not water costs.