It's usually more expensive to mix your own. My opinion of that potting soil is that it could be great or it could be hype. It's hard to tell. It uses cottonseed husks, which are great, but I personally avoid them because cotton is one of the most spray plants in the industry (sprayed as in sprayed with poisons), as a result, it's a lot harder for cottonseed to be OMRI certified. I also noticed they use phrases like Organic Compost, but then don't say if everything else is organic.
If it's OMRI certified, I'd go ahead and take it. Otherwise I'd pass on it. A good cheap soil that I like, it's sold on the west cost and in some parts of the east coast, is Kellogg Patio Plus. If it's available I'd look into it. It's $4 a bag and has worm castings, mushroom compost, kelp, chicken manure, and bark.