2.8 Suspected head gasket.

G'day all.

Seems my 2.8 has done a head gasket. There has been no signs of anything happening over the space of a week, which was the last time I did a proper once over onte vehicle.
I noticed an oil leak on the bottom of the sump. I thought I'd give it a clean and have a check of the fuilds.
To my surpise, the overflow bottle had oil leaking from the seams. Opened up the radiator. What used to be green coolant is now home to a thick black sludge.

I'm dropping it off to a mechanics on Monday.

Question: what price should I look to be paying to get the head gasket replaced?

Cheers.
Skinner.