May is the cheapest time to fly to Australia. Assuming May travel, the follow is true:

Airfare from LAX (or San Fransisco) to Sydney and back: 950USD.
Three or so days in Sydney.
Car rental for drive down Australia 1 (the coastal highway) from SYD to Melbourne: 50 AuD (35USD), split 4+ ways. (Or by train from SYD to MEL, 75AuD (60USD), so add 25USD to totals)
Two days in Melbourne.
Train from MEL to Adelaide: 60AuD (45USD).
Three days in Adelaide.
Train from ADL back to SYD (diff. route): 225AuD (180USD).
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Total cost of travel: 1200USD.
+ Hostel stays, 8 nights, 25AuD (17USD) / night: 136USD.
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Planes, Trains and Beds total: 1336USD.

Add in admission to the Sydney aquarium and trains in the cities themselves. And food.
Another, say, 164USD, say, for the sake of math.

Spending 12 days seeing three of Australia's largest cities (and a chunk of its countryside) with some of the coolest people you know? 1500USD, apparently.

If you want in, let me know and we'll start hammering out exact dates.