I have a mate that makes and sells his own batteries. He is in Essendon if you are anywhere near by.
You can get a Heavy Duty battery that has a minimum of 520 CCA from him for $100. I bought one off him 6 months ago, and that was the specs and price then.
He imports and puts all the batteries together by hand, and they are of realy high quality. Not the mass market stuff from big stores. Every one i send to him now swear by his batteries.
And I dont think anyone will be able to tell you where to get a quality 520 CCA battery from for $100.
If your interested let me know, will have to get the address for you. I know where it is, just not the exact address.