I'm quoting historic Melbourne pricing here; and don't know if it directly translates to Brisbane pricing.
Nov 2008 - MEL to LAX: $1050. Qantas direct
June/July 2009 - MEL to SFO: $930. Qantas via Sydney and LAX
Oct 2010 - MEL to LAX: $1090. Qantas via Sydney
Jan 2011 - MEL to LAX: $1150. Qantas via Sydney
Nov 2012 - MEL to SFO: $1150. United via Sydney and LAX
Sep 2013 - MEL to LAX: $1300. Qantas direct
I would have bought the flights around Australia Day when Qantas goes on sale. For Sep 2013, the sale price for MEL to LAX via Sydney was $1190.
I don't know if the Brisbane travel expo will have pricing to match. Certainly the Melbourne travel expo have had similar pricing to the above on carriers such as United, Virgin and AirNZ and some of the other smaller players in the region. Good luck with playing chicken with the airline carriers. I hope you get a great price!