I had trouble finding a really cheap one - also for my 9 year old to take on camp
I tried one of the large city camera stores and was advised that they didn't carry any under $100 as the quality would be dubious.
There are of course super cheap ones at places like Big W - I'm talking the $20 mark - but they looked like plastic rubbish to me.
In the end I found one on sale at Officeworks. It was around $50, but then I had to get a memory card and a little bag/case for it, so the cost did add up. It was worth it in my DS's case as he was already using mine and so I knew he'd use it.
Good luck with your search though!