Dick Smith Electronics stores have a big clearance sale on at the moment with some very cheap games across all platforms. If you've not picked up the Movie Transformers game this might be a good chance but stocks seem to be very limited.
DS Autobots game $12.50
DS Decepticon game $12.50
PC DVD version $7.50
PSP version $12.50
PS2 version $30 (only format not on sale)
PS3 version $30
Wii version $20
Xbox 360 version $20
Edit: Check http://www.dse.com.au for current stock availability so you don't waste a trip.