Depends on how far down you want to go. Are you doing it yourself or having it done. You just going to replace the head gasket or do other things while you're in there.
The head gasket itself is under $50. Labor and misc to just do the head gasket will probably be $300-$500, depending on what you do.
You can do it yourself and do a total stock rebuild for $600.
FYI...Head gaskets can be replaced without removing the engine. Difficult, but possible. That alone should save you some money.
I've got an 82 280zx that probably has a blown head gasket (unexplainable overheating). I don't have the means to do anything about it and am looking to sell the car. the body is there but well driven, the engine is great other than that. If interested let me know, we can work out a deal.