I'm fairly new to the dio scene, having built only 3 of them, one of which is worth actually looking at :)
Along the way, I've found a great (read: cheap) way of making groundwork and fake grass.
For groundwork, I collect earth from my backyard, allow it to dry throughly, grind it up, then sift through it, so I'm only left with a very fine dust. If you wish, you could add small pebbles, sand, etc for texture. In any case, I'll make a 50/50 (you can experiment with other ratios) mix of water and white glue, then mix this with the dust to effectively create mud. You can spread this and paint when dry, or just leave as is. Once dry, the glue keeps the earth bound together and hard.
Also, for fake grass, I'll just go to a Home Depot and get a garbage bag full of sawdust (they'll give it to you for free). Then you can mix this with paint and water to dye it. It takes forever to dry out, but the resulting 'grass' works great.
Just thought I'd share my ideas on this.