When we did it we averaged $992/month all up on travel. That doesn't include back home expenses like the various insurances that we still maintained nor the rates on the renter anything else. It did cover all cost associated with the cruiser, camper, kids education, internet connection etc etc as well as regular food, fuel, entertainment type things.
We were Mum, Dad, 13yo pink one and 10yo blue one. Costs varied depending on the week. What we did, where we stayed etc.. there is no general number that can be applied however, what I can tell you is that the consensus around the campfires that I found myself at during 2009 was that $1000/week is normal for a family of 4 and generally a number somewhere near $750/week was what the grey nomads reported.
Now, this may be a result of the sort of campfires I attended too. People who travelled like us, people who we got to know. It is very possible, in fact probable that there are whole other lot hidden down goat tracks that spend nowhere near that and have a great time doing it.
It depends entirely on how you travel, what you like to see, the vehicle you drive (the ever increasing cost of fuel will impact this discussion), what or where you like to eat, grog (we don’t drink much nor do either of us smoke), how long you spend out bush as opposed to hanging out in paid accommodation. Even the sort of accommodation makes a huge difference.
In the big town/cities you can camp powered from say $25/night to $50/night depending on where you stay. We tended to stay in the cheapies for 1 or 2 night stays and the dearer ones for the extended stays. That may seem counterintuitive but we figured if we are there for a week, we, well mainly the kids, would get the $50/night value out of the flash pool, the fancy games room and the big blow up bubble. Short stays, particularly O/N stays, were (if free wasn’t an option) the cheapest of the cheap. In at 5pm, out at 8:30am doesn’t allow you to get the most out of a good park. It was just a place to hold up on the drive.