Depending on if it is a conv. or a coupe will change the pricing dramatically (coupe being more expensive). Prices can range from $1,500 - $2,400. But also consider that there might be other issues that caused the head gasket to leak (defective radiator, water pump, thermostat, ect...), that will add additional costs to repair the vehicle. If you have any other questions, please feel free to ask.
Depending on the engine, it could be a little pricey, or a major investment. If the head(s) is/are cracked and/or the block is cracked, you could end up sinking the cost of the car into engine repairs. Buyer beware!