Since I used scrap-wood and earth already available to me, I only had to purchase a few items. Total Cost: $30.00
Rather than focus on building the actual structure, this instructable covers the cobbing part of the project.
This project was spaced out over two months to give each step plenty of time to dry, settle, harden and adjust.
Step 1: The Structure
The living roof is around 5 inches deep, with a layer of roofing tar at bottom (dried properly), thin layer of gravel on top of that, a layer of old cotton rug found at the local thrift store on top of that, and finally the soil, I have 2x4's spaced out to help hold the soil in place (you can just make them out in the photo). Also, plenty of drainage along the edges for excess water to run out of. Note that there is plenty of roof hanging away from the house all the way around.