I have begun with modeling the plant containers, for which I'm using OpenSCAD.
Here's a picture of my current progress:

The plants, soil, and watering tube are in the image to clarify usage.

Finished modules will contain: (not a final BOM yet. Just some thoughts)

• ~ $0.30: a small LED indicator (probably a red/green LED),
• ~ $3.50: a microprocessor (probably an atmega, not sure which one yet)
• $???: connections for power and I²C (I've had problems finding good electrical connectors for this application. Any ideas?)
• ~ $4.00/m: LED strip(s) to provide light (likely red/blue)
• Irrigation connector (likely normal 1/4" irrigation connector and tubing for prototypes)
• ~ $2.00/m: Irrigation tubing (probably silicone)
• ~ $1.50 Soil humidity sensor (the price is for one you'd find on amazon. I might be able to integrate this to the board [and seal it off with silicone], thus reducing costs.)

I'll post the .scad files eventually (when I have figured out the mounting and connection methods I want to use and have integrated those into the model.)

The container will likely be 3d-printed for the prototypes. Injection Molding would be preferable (after optimization therefore) for a larger-scale run.