SK187, You have to like the color of the dye (that makes it worth the money or not worth the money) and also some dyes do not have the correct pigment to block the correct wavelength of light. I'm not sure if every dye lasts as long either which also contributes to its cost by means of repeat dosing.
I went to the same store as you but purchased the Midnite blue brand for about $30/gallon. My first attempt was to put about 1/2 gallon in my .25 acre puddle. So far I like the color, a bit of a blend between blue and black and not much green. It seemed to last only about 6 weeks and my puddle is back to being coffee with cream color again. This late in the year I may just save the other half a gallon for next spring.