Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Newark Airport to Decatur, Illinois as you normally would.
For a flight from Newark to Decatur, you will be flying into Decatur Arpt. Decatur Arpt (DEC) is just 4.7 mi from the center of the city. Decatur Arpt is the only Decatur airport and it handles 0 inbound flights from Newark per day. 2 airlines have flights to Decatur Arpt from Newark on a regular basis.