Probably more than the car is worth, unless it is in pretty good condition.
The part itself is cheap [under 30$] but its the labour that kills you. They are going to want to machine the head, and machine the block, flat. Probably convince you to rebuild the head too, etc.
If you JUST want to replace the headgasket, your still going to probably have to flush your oil and coolant systems.
I'd say over 1000$ most likely, even just to fix it the cheaper way.
