Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Indonesia to Mongolia as you normally would.
This is defined as Denpasar Bali Ngurah Rai Airport (DPS) and Ulaanbaatar Chinggis Khaan Airport (UBN). Codes like Ulaanbaatar Chinggis Khaan Airport are often used to just call airports uniquely. 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.