Don't listen to anyone who says to use potting soil, gravel, or pool filter sand. Dirt might work for the short term, but when you want to rescape your tank becomes a mud puddle. Gravel has zero nutrient value and most of the time plants don't stay planted in it. Pool filter sand is great for keeping plants cemented it is inert and it will eventually turn anaerobic. You will thank yourself later if you put in the money now rather than later, most higher end planted substrates will last upwards of 10 years due to the fact that they attract and absorb the nutrients from the water column and make them available to the roots of the plant.