It's insane what the Amazon markup is on potting mixes.

Take your basic Sungro Black Gold bag (16qt) - it's $10 at Ace Hardware vs $21 on Amazon. So if you wanted to grow ~5 plants in a 4x4 tent - you'd spend $50 at your local hardware store, or you'd be paying Amazon $100+

It's even for worse ammendments, compost and other things you can pick up in bulk if you rent a pickup truck or cargo van for a day to take a bulk shipment. It's the paying people to bring you heavy things that costs a fortune.