A breeder block of Repti Chip is likely cheaper in the long run than buying pretty much anything from a pet store, and is a good substrate. Attentive spot cleaning can extend the useful life of substrate quite a bit, too.
I'd be careful with using substrate that might cause other complications depending on the exact situation. For example, if you're using a fish tank or other screen top enclosure, then a substrate that doesn't hold humidity might make the tank dry enough to cause problems. Just something to consider.