Actually, no it's not really that expensive. I just looked up pumps similar to the mist kings, and you'll see prices around 50-90$ just for the dang pump itself.
Mistking is a pretty great deal if you want a no-failure attempt at a misting system. Either that or botch one out of DIY stuff, but I have only seen a FEW posts that have DIY because everyone loves reliability and cost of the mist king. They last for years and the customer service is awesome.
just my thoughts. sometimes diy can fail and be expensive