It really depends on what you want for your substrates, and where you get it from. Obviously you'll be wanting to look at sand or gravel. If you go to a building merchant, or a garden center, you can normally get sand or gravel for a very reasonable price. I have found that fish stores tend to charge a lot more for the same material. However garden centres tend to sell one type of sand or gravel, if you want something a little different, then you might have to get it from a fish store and pay their price.
What would look good really depends on what you like. I like black or white substrate, preferably sand. I'm not really keen on normal straightforward gravel. I have found that the darker the substrates, the darker the tank, white substrate coupled with good lighting will result in a very well lit tank.
Your tank is fairly big, you probably need a few sacks of sand/gravel. Like I said, if you get it from a garden center, it's not too expensive. The price goes up considerably if you get it from a fish shop.
