As you can tell, it's going to be difficult to nail down a hard number on costs for a tank. If you're only worried about food and salt that will actually be a pretty small number. A bucket of salt (Reef Crystals) runs me about $45 but it lasts nearly 5 months if I only have to do regular water changes. I feed Formula 1, 2, and Prime Reef, Cyclopeeze, DT Oyster Eggs, Oyster Feast, and nori. If I picked up all of those at once I'd probably pay less than $100 but again they last for quite some time. Assuming that they each ran out at the same time, which they don't, I'd probably have to spend that amount every 3 months or so. These are very general numbers but I don't think they're terribly far off the mark. As far as costs go you'll spend much more on your animals, equipment, and in time spent working in the tank than you will on food, salt, and maintenance items.
Mike