None of my PCs these days have an optical drive but I wanted a spare one for those emergency situations or Windows installs. Surprisingly the cheapest branded USB DVD drive was this Liteon USB DVD-ROM drive I found on the JB HIFI online store.
Officeworks have the same item for $54 but theirs is also online only so they might not be able to beat JB by 5%.
+1 although I'm looking for an ext. DVD writer :)