DI water and soft water are not the same thing. DI water reverses the polarity of the ions in the elements that make the water hard so that they do not have their magnetic attraction. Soft Water is a chemical reaction that changes the chemical composure of the hard water elements, but they are still there, and can still spot things.
DI water can come two different ways. C/A tanks, where first they tanks take out the Cations, and then they remove the Anions. They also come in Mixed bed tanks, these have the different resins mixed so that you only need one tank at a time. I run two mixed bed tanks in parrallel, because I have found that they last much longer that way, and I get better quality water. He sizews usually are listed as 100, 160, 320,etc. The 100 is one cubic foot of resin, and is about 10 inches in diameter, and 3 feet tall. The 160s are 1.6 cubic feet and so forth.
As far as costs, it runs me around a nickel a gallon, or less. But it all depends on your supplier, and how much they charge you. I use 160's for my water, and get them for $37.50 a piece in a mixed bed tank.
