Large tanks are expensive to operate. The $100/mo
was just a quick estimate based upon what I have seen from my own tanks. I have two tanks one a 80 gal
tank and one a 75 gal
tank and those to tanks alone account for about $50/month in electricty. While mine are reefsetups with high output lights a large tank would need large pumps. Large bodied animals will produce alot of waste resulting in either a very extensive filtration system or frequent water changes. Those water changes require water and unless you have a well you have to pay a water bill for your town. So thats money plus the cost of salt.
Its nice to see a young person so energetic about the hobby but I also feel its important to start small and work your way up. If this is your first saltwater tank then I would suggest a 55 or 75 gal
tank to start with. If your saving up you can start with this size and it will give you a fair amount of flexability in doing fish only or reef. And if you go with fish only you can elect to do a preditory type setup or a more docile setup.