JB Hifi have Disney Sing It for XBOX 360 on clearance for $19.99
It comes with a wired USB microphone.
Game + mic for $20 is a great deal by itself, but trade the game + Mic into EB Games for $38 (plus any Edge or Edge+ bonus) and you are getting a very very good deal.
Update:
EB have dropped their trade-in price to $24 following an overwhelming response on Tuesday from OzBargainers!
arbitrage opportunity will disappear soon.