Home Depot offers the Miracle-Gro 1.5-Cubic Foot Flower Garden Soil Bag for $6. (ACE Hardware has it for about the same in-cart with free in-store pickup.) Opt for in-store pickup to dodge the $79 shipping charge. That's the lowest price we could find by a buck. It feeds plants for up to three months.

Update: The price at Home Depot has increased; however, it's still available at ACE Hardware for the in-cart price of $5.99 with in-store pickup.