Stumbled across this today! Thought I would share it here since I know a few of you have migs.
These guys are selling MIG gas bottles so you can buy them outright and just exchange when they are empty, saves that pesky $18 a month for rent from BOC.. Now just have to figure out what I need to weld up to use up the rest of my BOC bottle..
http://www.gasweld.com.au/