We stayed in the Big 4 Cairns Coconut Holiday Resort for a few nights on our way back from up the Cape.
That's just down the road from our house. It's expensive, yes, but for kids it's a wonderful place. Friends of ours book an unpowered site for the night there, but don't actually bother about taking their tent. They use the facilities all day - kids have a ball. Then go home for dinner and to sleep, and come back the next morning for a couple of hours play. It's a cheap water-park weekend for them, and the kids have a blast. They don't do it every weekend, but occasionally it's great.
I don't think it's just the Big4 parks that are expensive, although they're probably the most visible. We've stayed at three independent places in the last 9 months - two proper caravan parks (one in SEQ, one up here) and just a general camping spot. Each of them has been over $30/night for a basic, unpowered patch of grass. A powered site is usually only $5 more, which I find confusing, but I guess you're paying for the floorspace, not necessarily the power.
I suspect it's just the way the market is going. If people will pay the $30/night, then as a park owner you'd be silly to charge less. Their costs are probably going up all the time as well.
Thanks!
Matto