It varies immensely depending where you are and when, there are a lot of places you can camp for free, with toilets and even a shower, but if you stopped in a holiday resort style camp site over Christmas ( which is peak, peak season) on a powered site you could pay $50 a night. Normally you'd find a good place somewhere in the middle and when it is peak season the hostels are also expensive and full.
We would camp in the free sites for 2 or 3 nights then stopped in a resort style for 1 so we could refill our water and enjoy the pools etc. we used a portable battery jump starter pack to power 12v lights and a little tv and found having a pop top was great as a lot of people with converted vans couldn't walk around or cook indoors on bad nights like we could. There are a lot of places where you can park next to the sea, wake up in the morning and enjoy the view on a deck chair with your feet in the sand. Priceless and free!