biweekly is a stupid word. people mean water changes every other week, not twice a week.
I have a 55g tank, and at 10% weekly (I think stable is better) I spend $75 a year on Instant Ocean Reef Crystals. I would guess a 100g would be about twice that.
Salt is important cause it is like the ocean dehydrated, so you have all kinds of trace elements in there that the animals in your tank need. Like vitamins for the coral and fish. That's why you want to use pure water, so that hardness in the water or other impurities don't throw off the balance.