Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Surabaya to Illinois as you normally would.
For a flight from Surabaya to Illinois, you will be flying from Juanda. Surabaya Juanda (SUB) is just 15.0 km from the centre of the city.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Surabaya Juanda Airport to Chicago O'Hare Intl Airport flight would often be abbreviated as a SUB to ORD flight. Chicago Midway Airport (MDW), Moline Quad-City Airport (MLI), Greater Peoria Airport (PIA), Bloomington-Normal Airport (BMI), Champaign University-Willard Airport (CMI) are other airports near this flight route and their unique identifiers/IATA codes.