tank, lighting, sump, skimmer, heater, media reactor, water treatment/filtration etc.
The initial hardware is very expensive, and the cost is directly related to the size of the tank.
After that 1-1 1/2Lb live rock per gallon, and the price of livestock, plus test kits, plus, plus....$$$$$
Starting with the largest tank that you can afford is an idea that is fading in popularity.
I started with a 5 gallon. The per gallon cost is higher, but the net cost is lower, and it is a safer way to find out if saltwater is right for you. It forces you to do your research about species compatibility. It forces you to get accustomed with the husbandry of the saltwater tank. But they are nowhere near as difficult to keep as many people would have you believe.
If you find that you love saltwater you can always upgrade. I'm planning my new tank, a shallow lagoon setup in a 40gallon breeder. But the 5 gallon was a great experiment (a sandbox if you will) into what I like , what I don't, what works, things to improve, etc. and it was approachable and affordable. Much better than pouring thousands of $$ into a new hobby to decide later that you don't have the passion or the time for it, just to end up as another "getting out of the hobby everything must go" posts in the classifieds forums.
In every other endeavor that you do people always tell you to "start small", and keeping a reef tank does not have to be different.
check out:
http://www.nano-reef.com/articles/
The beginners articles are very useful.
BTW I'm helping my room-mate set up his first tank. 2.5gallon with a HOB refugium and a 18watt PC fixture. Probably stock with some GSP, Zoas, and mushrooms, a blue leg hermit and a snail or 2
The tank is cycling with rock right now and the cost to date is Can$85 for all of the hardware, and Can$18 for substrate and live rock.