Maltodextrin is the primary ingredient in all competent sport gels. It is a simple chain of glucose molecules. Glucose (aka Dextrose) is your body's preferred source of fuel. Unlike more complex chains, known as starches, maltodextrin can be metabolized almost as quickly as free glucose.
Raw maltodextrin is rather amazingly cheap compared to commercial sport gels, like Gu. I bought my last batch at a local brewery supply store, Fermentation Frenzy. I paid $1.95 per pound. That works out to about $0.11 for 100 calories. This compares to $1-1.50 for Gu.