First, are you certain it is the head gasket and not the intake gasket?
Labor for the 3.8L is 11.8 hours to replace both head gaskets.
And 9.8 hours to replace both head gaskets on the 3.4L.
So, you can figure roughly $700-$1,000 labor and around $250-$450 in parts, depending on the shop doing the work and how complete a job they do.
One thing that I will caution you about.....................
If coolant has gotten into the oil pan, I would seriously think about dropping the pan and replacing the main and rod bearings. If you don't, they will fail eventually.
If this vehicle has a blown head gasket, I would personally walk away from it.