Your costs of material and reasonable labor form the basis of your prices. If you're producing the product rather than reselling it you have a bit more flexibility in how you value your labor.

I buy based upon need for variety not found in the normal retail outlets but the item must be clean of weeds and properly watered in an appropriate sized pot for current root development.
You won't compete with the big-box outlets by price.
Factor in also the cost and nuisance of water hauling to keep the plants in top shape.

You might also consider offering quart size single plants since summer heat seems to be starting early this year. Tougher on the tiny rootlets and young leaves of plants usually offered in standard 6pk.