Winterwhite, I am in Melbourne too, the thing is with those grids thing, you have to try quite a few bunnings to get them and each bunnings priced them differently.
I live in Parkdale and the nearest Bunnings to me is the Mentone Bunnings. They are selling them for 11.50 each for a pack of 5. (IBEX GRIDS). That is the cheapest I can find. Apparently, the more lousy the surburb is, the cheaper it is. More affluent areas's Bunnings sells them at like 15.00 dollars. I myself have seen several prices.
So, try the Bunnings over at the west side. You might luck out. You wont find them at ALDI and Reject Shop anymore. Even Big W stops selling them.
As for corflute, (broken link removed)
go to this guy on ebay. Most of us all buy from him. He is a great and friendly guy and he got tons of corflute supercheap. He is located in Keysborough. Can be too far from you if you do not have transport, but seriously, it is the cheapest corflute you can find. a 190x130cm can fit a 2x5 easily. You do not even have to buy 30 pieces. Just message him an offer and you can buy it in smaller amount.
Hope this helps!
BTW, ACS (Australian Cavy Sanctuary) Lakes Shelter, QLD, is having a renovation right now. Alina, the shelter manager said she has tons of 2nd hand grids for sale. She is selling a pack of 5 for 13 dollars.