hard to say exactly because:
1. We don't know your zip code and labor rates can vary widely around the country
2. There may be incidentals to this repair, such as new coolant, belts, hoses and in most cases, the radiator should be replaced if the head gasket problem was caused by overheating. You may also have to have the cylinder head "trued" in case of overheating.
The short answer---if everything goes well---from $500 to $800.
If things go badly, and you need cylinder head work, it could run up to $1200 and more.