Inert substrate and expensive aquarium soil aren't the only options. Try this:
Start with a layer of dried leaves, such as maple, oak, or hickory. Avoid waxy leaves, conifer needles, walnut, and tree-of-heaven. Obviously you should collect these from a clean area well away from a road, cropland, or other toxin source.
Add a layer of ordinary woodland topsoil (again, from a clean area), or the cheapest bagged topsoil you can find (Hyponex works great).
Plant your plants.
Top with a layer of gravel.
Fill with water.
Run your filter with just polyfil for a day or two to get the suspended silt out, then put in your normal media.
It's dirt cheap (hah!) and works great.