This is a little off topic for kitty litter, but related to being poor. For a cheap nutrient rich substrate, I would do what I did with my first tank. Soil from a wooded area, mixed heavily with Vermiculite and water so that it is a thick paste. Cover the bottom of the aquarium with this, then a few inches of gravel on top of that. The drawbacks to this arrangement are that if you uproot a heavy root feeding plant, you will instantly cloud your water. Planting can also be a chore. And over time, gravel will get pushed down into the soil and vermiculite mixtureand you might find yourself adding more gravel over time. You pretty much must get it right from the beginning. (Don't think this kind of stuff won't happen with kitty litter after it gets all muddy though)
The "gravels" that I have experimented with are:
Pea gravel from Home Depot. 40 lbs bag for under $10 (USD). I woudn't use it again as it is a bit large and getting fine rooted plants and stem plants to stay in it is a challenge.
Tex blast sand from a local sand blasting supply store. 100 lbs bag for $5 (USD). I wouldn't use it again as I seemed to have a horrid time suctioning it and I didn't like the brightness of it or the singular color.
Tex blast #2 gravel from the same place. 100lbs bag for $6 (USD). This is somewhere between pea gravel and sand both in size and color. Very nice stuff. Perfect for planting.
Of course you could just go with a pure gravel substrate and just let the fish fertilize your substrate. I'm going to try this when I get around to putting some water in one of my 10 gallons.
As always, YMMV.