I'm not in the business, but I know this can be a little more expensive than you think, depending on the style of fixture, type of bulb, etc.
On my pool, the bulb is about $28 and the required gasket is about $36 (pool store prices). The time to disassemble and replace the bulb can be an hour or so. My fixture is a Pentair brand and it has 10 screws that have to come out to remove the lense (to change the bulb). If trouble is encountered removing the underwater fixture due to a stripped or frozen screw, the time requirements can go much higher.