The price differences that I found were not as large as I expected, but the travel time involved was very different.
The following are the cheapest options for travelling in two days time, with a return 8 days later.
Greyhound Coaches
Bus: Brisbane to Melbourne $156.33 with a travel duration of 24h 45m
Bus: Melbourne to Brisbane $156.33 with a travel duration of 23h 25m
CountryLink Trains
Train: Brisbane to Melbourne $147.67 with a travel duration of 27h 46m
Train: Melbourne to Brisbane $147.67 with a travel duration of 34h 35m
Jetstar and Virgin Airlines
Plane: Brisbane to Melbourne $214.50 with a travel duration of 2h 30m
Plane: Melbourne to Brisbane $166.50 with a travel duration of 2h 10m
The result: You could save $100 or so on the return trip, per person, but at a cost of an extra 40+ hours travel time, for the round trip.
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