I think crimzy is pretty spot on. For a large, full reef, 180gal+, it definitely costs a good coin. This is why most reefs are 120gal/under. With FOWLRs, the tendency is being able to run the tank cheaper, which allows a bigger tank from the get go, which is able to house typically more desirable fish.
Perhaps this is way I find predator reefs to be the among the most fascinating tanks out there.