I'm not a professional mechanic so take the info with a grain of salt but I'd say it would be tough to guesstimate the cost involved because of so many things that could need fixing. How did the head gasket blow in the first place ? why did the car overheat ? if it was a stuck thermostat that's cheap, but if it needs a new radiator, waterpump, etc then those things will need to be taken into account. Headgaskets don't blow on their own, unless its a manufacturing defect/bad-design (Mk3 Supra, Neon etc).
Then you have to take into account how hot did the car get and what it damaged, if it didn't warp the head too bad you can typically clean and slightly deck the mating surface and be done with it. However, if the head is warped then it'll need a new head.
Then you have to take into account the mileage on the motor, if its got over 100k miles and you're opening the motor up you might as well get a valve job done on it...cost would vary on this too depending on what needs replacing.
I'm not trying to scare you but just wanted to inform you on things that could be affected. If you don't care about any of those or if the car only needs a head gasket removed and replaced (R&R) then the labor charges can vary from $500 to $800 here depending on the make and model. That's just my guesstimate...
I'd call around and ask your local mechanics (anything between the really cheap quote and the dealer quote would likely be the avg cost).