This kind of thing is batter to look at actual site then only can know what is the price, coz that may many way to do so. If the "fake 2nd floor" is not make any changes on outlook, that mean usually are using, metal frame with wood base for the floor, and i think that not really able to fit in a bahtroom, however, if that work related to concrete, then have to check your existing stucture example beam and column was it enough to hole another floor. as you can understand is, taller building need bigger culumn and beam. When it is not enough, that mean u got to break everything and re-do from footing, which just like Drian siad, cost extreamly expensive compare to buy a new double stories.