[linux.conf.au][1] (LCA) has been running 10 years now and draws over 500 attendees.

This is the pricing table from the last LCA:

> Professional / Hobbyist / Student  
> $795.00 / $375.00 / $125.00

Open Source Developers Conference (OSDC) has been running 6 years and draws closer to the 300 mark. From memory tickets were $325 aud.

Web Directions South has attracted over 500 attendees since 2008. [Pricing info here][2].

I should note that LCA's numbers are high because many attendees fly into the country for it. 

Sydney vs Melbourne is tough. Both have ample facilities to hold a conference. Sydney's population is 4.5M vs 4M for Melbourne.


  [1]: http://en.wikipedia.org/wiki/Linux.conf.au
  [2]: http://south10.webdirections.org/pricing