Store bought potting mix is one of the worst mediums to grow in. Its got fungus, weeds and pests in it and the nutrient mix usually sux. When it comes down to it, you get what you pay for. If you are only going to spend $10 on your plants from seed to harvest they aren't going to give you shit back. Better Medium/nutes = better quality and more growth, quicker. I tried doing it the cheapest way possible with $2 buckets, cheap potting mix and Miracle grow. It works, but all it will give you is schwag.