How much should I expect to spend on a Head gasket job on a 2000 Mercury Sable?

Which engine, 3.8 liter???? This is the one that usually has the trouble. It is usually caused by a failure in the engine temperature switch. The cooling fan does not go on at the right time and causes an over-heat, warping the heads. To answer your question, you can fix the problem yourself with a little gumption and know how from a Haynes manual, or pay much more by having someone else do the work. I would charge about $1500 for this repair, including parts and machining. Good Luck!!