".....is CASA generally helpful and cooperative in helping people in this kind of venture...."
Oh, yes. Definately! They'll be falling over each other to assist you!
Basic AOC? My guestimate: $7,000 plus and at least nine months of frustration.
In a different time I once got a charter AOC (for pressurised turbine aircraft) in less than 15 days! But those days are long since gone.
Check out another current thread where one operator has been trying for weeks just to add a new type to his AOC.