Darwin-Bali and Darwin-Manila flights are advertised as available for $99 while Darwin-Singapore are advertised at $109. In reality Jetstar's booking engine has Manila available for $95 while Singapore is $104.
Return flights are cheaper due to lower airport/government charges. Bali-Darwin is available for $30.76, Manila-Darwin is available for $29, while Singapore-Darwin is available for $62.31.
So that's from $125 for an international return trip - as long as you take your own food and drink, don't check in luggage and can avoid Jetstar's booking and service fees!