When I went to the Gold Coast, hostels were around $25/night... booking ahead was recommended since it was around schoolies, but most of them don't stay at hostels anyway, and it's not going to be an issue when your'e going.
I ended up staying at the Surf n' Sun in Surfers Paradise, and as far as partying goes, they always had these specials (i.e. $5) for all-you-can-drink punch followed by free cover at a local nightclub. Best bet was to buy a carton of beer and hang out by the pool and find out where everything cheap was to be found... Which generally is how it worked out for me pretty much anywhere along the coast.
Other than "special" days (i.e. my friend's wedding, me & a friend's birthday, time set aside for shopping) I don't think I spent much more than $60-$75 a day, including drinks. Find out where the specials are!