It's hard to find any chlorine source that's cheaper than 5 gallon carboys of 12.5% Liquid Chlorine. It's equal to 10 1/2 gallons of 6% Bleach. Carboys run $17, so it's like paying $1.65/gallon of Ultra Bleach. Doesn't get much cheaper than that for chlorine unless it's being given to you.

Carl