It's more expensive because it's significantly harder to get than the other wools.
Isn't light-blue dye just lapiz lazuli + bonemeal? Magenta Wool sells for $250 and it uses magenta dye wich requires two dyes: pink dye (rose red + bonemeal) and purple dye (lapiz lazuli + rose red)
Both ingredients for the light-blue wool are present in magenta wool along with the addition of two rose reds which means either the price for light-blue is wrong like I initially posted, or magenta should be more expensive.