Steam - 50% Off Borderlands This Weekend - $25AU
The best price I can find here on disc for PC is over $70!
I've still got Borderlands sitting wrapped waiting to be played on 360 so I can't comment first hand about the gameplay, but it does get great reviews, ment to be alot better in Co-op though so good if your friends can grab a copy too.
(Note that the 4-pack is also reduced. It's £44.98 this weekend...about $75)