GAME have cheap game for DS and Wii, some of them are $8.
According to http://www.ecogamer.com/content/view/1297/6/
Geometry Wars: Galaxies - $8
The Mummy: Tomb of the Dragon Emperor - $8
Teenage Zombies - $19
Lunar Knights - $19
Undercover: Dual Motives - $19
Neves - $19
Need for Speed: Carbon - $19
Kira Kira Pop Princess - $19
These can be confirmed on the GAME website. All games listed below are below $20
DS
Geometry Wars: Galaxies (DS): http://www.game.com.au/viewtitle?code=GEOWARSDS
The Mummy: Tomb of the Dragon Emperor (DS): http://www.game.com.au/viewtitle?code=2260VU
Teenage Zombies: http://www.game.com.au/viewtitle?code=NDS-0085
Luna Knights: http://www.game.com.au/viewtitle?code=13169
Undercover: Dual Motives: http://www.game.com.au/viewtitle?code=E104363
Neves: http://www.game.com.au/viewtitle?code=NDS-0080
Need for Speed: Carbon: http://www.game.com.au/viewtitle?code=NFSCADS
Kira Kira Pop Princess: http://www.game.com.au/viewtitle?code=NFSCADS
Konami Arcade Classics: http://www.game.com.au/viewtitle?code=13530
Jam Sessions: http://www.game.com.au/viewtitle?code=300005026
Godzilla: Unleashed: http://www.game.com.au/viewtitle?code=13410
PRISM: Light the Way: http://www.game.com.au/viewtitle?code=13606
CHICKEN LITTLE ACE DS: http://www.game.com.au/viewtitle?code=61446
Wii
Geometry Wars: Galaxies (Wii): http://www.game.com.au/viewtitle?code=GEOWARSWII
Xbox 360
Pro Evolution Soccer 6: $14- http://www.game.com.au/viewtitle?code=13111
Table Tennis- $19- http://www.game.com.au/viewtitle?code=1208X360
Enchanted Arms: $19- http://www.game.com.au/viewtitle?code=1001194
Madden NFL 06: $19- http://www.game.com.au/viewtitle?code=14945
TimeShift: $12- http://www.game.com.au/viewtitle?code=12496
Dead or Alive Extreme 2: $19- http://www.game.com.au/viewtitle?code=DOAEXT2360
Perfect Dark Zero: $19- http://www.game.com.au/viewtitle?code=XBS0151
Great for trade in deals.
Good prices, but of the X360 games (assuming you wanted to trade at JB), Timeshift is the only one you can trade. The others are all on the exclusion list. I cant find the list for EB Games so I'm not sure about them.