How much will it cost to replace the Head gaskets of Lexus ls430, 2001?

I found out that the radiator antifreeze is way low then I checked engine oil and detected that its color is just like mud so I assume that the antifreeze has somehow got mixed with the engine oil. I talked with a reliable mechanic, he told me that I need to have the Head gaskets replaced or I have to try the Head gasket sealant and pour it into the radiator..
I really need your advices.
Happy New Year!
Phil Nguyen