Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Surabaya to Mongolia as you normally would.
Surabaya Juanda (SUB), located 15.0 km from the city centre, is the airport you fly out of when you book flights from Surabaya to Mongolia.
This is defined as Surabaya Juanda Airport (SUB) and Ulaanbaatar Chinggis Khaan Airport (UBN). Codes like Ulaanbaatar Chinggis Khaan Airport are often used to just call airports uniquely. While you’re still figuring out your travel plans, it’s not a bad idea to learn the names and codes of other airports around these areas: Ulaanbaatar Buyant Ukhaa Airport (ULN), Dalanzadgad Airport (DLZ), Khovd Airport (HVD), Moron Airport (MXV), Ulaangom Airport (ULO).