Hey there
My husband and I were on a low budget as well but we managed to get a return flight from Brisbane to Perth for $450 each. (Booked it pretty early.) In Perth, we hired a campervan from standbycars.com.au and it cost us about $1200 for two weeks. You can only book them about three weeks in advance but if you don't travel right at peak season you get pretty hot deals.
There is also the clubredcars where you can hire a station wagon incl. camping gear for something like $350 a week. (Done that four years ago and it went fine too. Not as comfy as in the campervan

)
We didn't spend much money on accomodation as we went to a library first to get the book about free campsites and I tell you, you can camp at a lot of places for free or even find friendly people who offer you to sleep in their driveways. It looks like we are real tighta... but sometimes in life, you know, you can still travel with just a little money.
I did the Nullabour thing too. But the fuel cost is just too much. And I just know from Brisbane it takes you five days and night of driving. It's not worth it. You could think of the train. Perth to Adelaide (don't know to Sydney) was $620 incl. (one way) the car on the train and it took two and a half days.
Well, really calculate it through. I would say how I did it last time with the flight and the campervan was the best value for money.
Good luck!