1998 3.1 with internal coolant leak

My '98 Lumina with 147k miles has coolant mixed with oil. When I checked the oil recently it was a light brown color. Two weeks ago the heater was blowing cold and it almost overheated. I added some prestone and was planning on changing the water pump; thinking that was the problem. I checked the oil then and it was ok.

What should I expect to pay for the repair assuming it has to do with the intake manifold or head gasket?

Thanks for the help

An intake gasket replacement is going to run in the neighborhood of $650-$800, head gaskets $1000-$1100 depending on where you live. Make sure they replace the thermostat and the oil pump drive o-ring while they have it down, and make sure they clean the sludge out of the valve cover baffles.