Head gasket or something else?

[leasam]

I have an VS Series II commodore wagon.
I have an oil at the outer edge of the valley at the back of the motor and a pool of coolant in the middle of the valley.
I dont have any water in the oil, however there was a bit of brown sludge under the oil cap. Ive also had brown sludgy stuff in the radiator and flushed and cleaned it out today and while running there was bubbles appearing. I have also been experiencing a fair bit of water/coolant loss. Does this sound like a head gasket issue or inlet manifold gasket? If so what is generally the cost of getting a head gasket replaced?
Forgot to add that the car is not overheating - only get to about 1/4.