The words "cheap" and "Yamba" don't really go together - the town has become quite trendy and its' prices reflect this. I'd try Woody Point at Iluka instead of Yamba, or Flat Rock at Ballina. From memory Woody Point is about $15 a night and Flat Rock slightly more. Both are in fantastic spots right on the beach.
It's all fun and games until someone gets hurt - then it's bloody hilarious!