My current goal on this trip is to get to Ulan Bator, Mongolia but the Mongolian visa will use up the last blank page of my passport and I intend to keep travelling for a couple months more.
So yes I know I might be able to obtain a new passport at the Australian embassy in UB but I don't mind the restriction and am considering my options.
I'm interested in everywhere, the only restrictions are the generally cheapest flight, and the destination has to be a place where Australians don't need a visa, or where the visa-on-arrival does not take up a full page. (USA is also out as I happen to require a visa there.)
I know flight prices change and there are changing special offers etc so I'm not looking for the absolute cheapest, just the destination which should pretty much always be available and pretty much always be cheap. If the cheapest is still not super cheap it's still an acceptable answer.
Experience will probably provide the best answer since if none of us are familiar with flight prices out of Mongolia we'll be able to find current cheap flights on the flight search engines, but it will be harder to know for sure which of those destinations are usually the cheap ones and which are seasonal or special prices.
(For example there might be cheaper flights sometimes but generally the cheapest places to fly from Sydney are Kuala Lumpur and Singapore.)