Whoose, what are you seeing?
Like Ironwood said we don't know what goes into MG but at least organic has some "certification". I'm happy they carry an organic mix and +1 Worth's ferts. I posted pics of Costco's soil/mix price and ingredients, reply #21
here.
Here in Los Angeles, Costco mix prices out to a little more per quart than regular MG but much less than MG organic. Although we have what seems like as many hydroponic/garden centers as Starbucks, the prices are high for Promix and organic Fox Farm type mixes, so Costco gets my money. We have a lot of wholesale nurseries too but I can't do a huge bale, nor do I have the money for it.
I have tried to make my own in the past but dialing in the lime and my lack of patience means I don't give the mix time to settle its pH the way the big guys can do on their huge scale.