Mortein Indoor Protection refils are $16 for 152g can @ Coles
Black Flag, sold by Bunnings refils fit the Mortein system and are $9.98 for 152g can saving you $6 a month - $72+ / year.
Win
Black Flag http://www.bunnings.com.au/black-flag-automatic-indoor-insec…
Mortein http://shop.coles.com.au/online/national/mortein-naturgard-a…
I've looked at these, but can't quite bring myself to have insecticide constantly pumped into the air. It's probably no more harmful than the plug-ins that we sometimes have to use for mozzies and sandflies though, I guess.