Stayed in Lismore showgrounds a few months back. Lots of room, gates locked about 6pm but you can get key with deposit. Toilets/showers usable but very run down when I was there, but they were renovating. Check dates there for Speedway races some weekends. Was Cheap $10-!5 I think.
Cheers,
RambleGambol
If you want a caravan park, we stayed at the Road Runner on the Casino road. Mostly permanents, but a separate section for "tourists". It was much cheaper than the Ballina ones we checked out, and we enjoyed it. A light aircraft runway nearby was entertainment.