rail can be quite cheap but always check with the corresponding flights, especially now that we have 2 buget airlines now (at least im sure that we have an easyjet thats starting here soon if not already but i just could not find their website.
Anyhow If u like trains :
check out http://www.railpage.org.au/railmaps/
for a nice map of the train lines in OZ, to find out which route to take, and
for some info on rail passes,
according to http://www.countrylink.nsw.gov.au/
sydney to brisbane is $115 one way (adult)
(and takes 15 hours)
$89 if u book early enough (else $200) via virgin blue (25 min flight)
$75 early bird ( ranging to $201) via qantas,
for all the prices i put in a date of nov 16th, so u will need to check for the exact date closer, butit gives u an idea of price comparison.
hope this helps