instal cost depends on what you use and how you install them.
If you have access to the transom from inside the boat, installing surface mounts is just a matter of running a power supply line to the transom, drilling a hole above water line to get the wire thru.
on the outside, what you can do is fasten a piece of 3/4" starboard as a mounting pad for the fixture. 5200 plus a few screws will be fine. Then you attach the fixture to that mounting pad so that you can unscrew the fixtrue in the water if neded without having to worry about water getting into the screw holes. Run the wires up to the hole, seal and put a clamshell.
With good access, its 1 to 2 hours per fixture at the most.
obviously if you are to going to put some on the hull side, you can't really use surface mount, but the above will work well at the transom.
When i had the lights installed on the 70 footer I run, it was a little complicated due to the hull shape. I had to have the yard build a custom mounting block. Unfortunately, we coudlnt' get the wires up above WL because we have a fuel tank in the way, so we had to come in behind the lights.
then it got even more complicated when the yard failed to follow the instructions and apply sealant behind the fixture,. even though they were supposed to keep cooling passages clear. When they tried removing the fixtures, the wires ripped out on 2 of them. expensive mistake for the yard. Since we were ready to launch and i dind't want to wait a few days for replacement lights, I had them drill a hole in the mounting block so that we could coil extra wire in there, with a cover. Once i got the replacement fixture, I just dove in, pull the wires to the swimplatform, connected and sealed the connections then coil the wires back in the hole and screwed the fixture. simple. I can now replace those two without a haul out.
unfortunately, the one that failed is the one they were able to pull out to remove the sealant, it doens' t ahve the extra wire coiled behind
