How much money will it cost to replace pool light fixture?

The fixture alone will be $150-$300 with bulb, labor will be 2-3 hours depending on whether you drain the water low enough to work on it. If not the time will be determined by the time to remove that much water which may take hours alone! The exact cost will not be known until you begin doing it. An estimate is just that, often unforeseen issues are exposed once you begin the job requiring a revised estimate.