look at the white tufa rock-its not that pricey and stacks great in the water-it wont shift and kill fish when they trench as much as some of the other types of rock.
I use to really like tuffa rock, but it is really sharp... and if fish rub up they can get scratches. It also breaks down into small sand like pieces over time. You will have to buy more...
Just make sure that you like the holey rock. You can find it cheap, but it is still an investment when it comes down to it. Even if you get it for a $1 a pound, it still takes 80 lbs to fill your tank.