Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Jakarta to Mongolia as you normally would.
CGK and UBN 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. CGK to UBN refers to a flight route from Jakarta Soekarno-Hatta Intl Airport to Ulaanbaatar Chinggis Khaan Airport. Ulaanbaatar Buyant Ukhaa Airport (ULN), Dalanzadgad Airport (DLZ), Khovd Airport (HVD), Moron Airport (MXV), Ulaangom Airport (ULO) are other airports near this flight route and their unique identifiers/IATA codes.