Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from San Antonio to Decatur, Illinois as you normally would.
SAT and DEC are unique codes to identify airports used in the aviation industry. They are defined by IATA (International Air Transport Association) and are commonly called IATA codes. SAT to DEC refers to a flight route from San Antonio Airport to Decatur Arpt Airport.