They are easy to make, I've made and sold several. But, quite time consuming, and the more expensive ones usually have finer detail and better quality fabrics and lots and lots of tulle for a very full skirt. The cheaper ones that are usually for the crafty moms that are knotted tulle like the one you linked, don't tend to hold up as well as the boutique quality ones. Also, those crochet pieces can be bought for super cheap too.
Something like this... now that is high quality worth a higher price tag:
http://www.etsy.com/listing/83324360/beautiful-whimsical-...
So be sure to do your research. On Etsy, that particular style ranges between $20 and $45. Scroll through the listings, you can tell which ones are better made and poofier and note the price difference:
http://www.etsy.com/search/handmade?search_submit=&q=...