I have a 125 FW tank, and my only input to this thread is
NOTHING is cheap for a big aquarium.
I have over $100 worth of sand as substrate in my tank, little HOB filetrs for $40 are history as they wouldn't be worth fooling with on a tank of this size, I have a Fluval FX5 on this tanks and am considering adding another one.
I could only guess everything would cost even more for SW. so if you are on any kind of budget, be sure to consider the huge costs of everything used on a large tank...
Just my thoughts...