OK
You can buy a generic Op's Manual for about $700 or so, unless you are an instructor and love paper work a charter AOC would be easier.
Obviously you need to qualify and cover all the CAO 82's etc and then there are things like passenger carrying insurance for each aircraft which can get expensive.
I have heard some people buying exsisting AOC's but I can't imagine CASA approving of this and surely they would go through the complete re-issue process if this was done to cover themselves.
The ottom line is do you have enough work to make a go of it, CASA isn't going to issue AOC's for say 50 hour's flying per year, are they ?
I would think all up to get going just for the AOC it will cost about $2000 including the chief pilot interview and flight check for a basic AOC charter.
Cheer's