Especially when you can make your own for dirt cheap... my pots are all filled with a mixture of Peat Moss, Vermiculite/Perlite, and Compost, with some slow-release ferts and a little lime thrown in. If you have a special mix you normally use for plants, you can just fill the bottoms with this, because the peat is cheap in bulk, the perlite and vermiculite are relatively inexpensive and in most nurseries/garden centers you can find various types of compost for 1-2 dollars a bag, if you're worried about using your own.
Or, you know, fill it with old potting mix if you have it. More space for roots is always a better idea.