Kool aid is not expensive if you are dyeing a skein here and there, but for larger quantities, it costs much more than commercial dyes. I haven't dyed in a while, but it took a lot of Kool aid to get a decent colour whereas a little bit of Ashford dye got a better result. A friend of mine did a dyeing demo a little while ago, I can't remember what dye she used, but the price was still on it, I think about $2.50 for a bottle. She used about 1/32 of a tsp for a skein, and the bottle must have held several ounces. It takes, for me, at least a package + to dye a skein (4 oz) of yarn, in some cases several packages - so at least $.35 - $1.00+ per skein; I don't think she spent meore than a penny or so per skein.
I have no problems with Kool aid; it is safe for kids (as long as they don't drink it, blech), relatively cheap in small amounts, and easily found. But for people interested in dyeing significant amounts of yarn or fibre, there are better options.