In addition to cost (which I don't have numbers for) there's a question of efficiency: Geothermal heat it typically relatively "low temperature" heat, which makes for very inefficient power plants, especially in southern places like Italy and Greece, where there is little or no easy access to cold reservoirs (like the sea around Iceland).
Geothermal energy is the perfect source for heating cold places in winter, or otherwise heating places you want warm, but you need quite specific geological conditions for it to be an efficient means of producing electricity.