No flaming from me. That's a normal fish only tank *shrug*. Of course I'm still kind of a beginner to salt, so maybe someone else will come along and flame you, lol.
It depends on whether you want to do a fish only tank, a fish only with live rock tank (fowlr), or a reef tank. For any of these you will initially need to invest in salt, a hydrometer, and an appropriate test kit. (At the least nitrates, ammonia, nitrites, and PH just like FW...but nitrates and PH are much more important to monitor in SW than FW.)
A fish only tank is like santoury said - saltwater, fish, a filter, and decorations, like with a freshwater tank. In a fish only tank there will be nothing to eliminate nitrates, except for some algae growth, and nitrates may build up heavily depending on how often you maintain your filters and clean the cartridges. A lot of saltwater organisms are extremely sensitive to nitrates, so you'll want to go with some hardier fish that aren't.
This kind of setup is like santoury said, no more expensive than freshwater other than the investment in salt.
In a fowlr tank your primary source of filtration is live rock.
At the least you will need to invest in powerheads, good live rock, a heater, sand, and enough light to grow some coraline algae, about 1-2 watt per gallon. Good live rock is around 6 dollars a pound and up (around here anyway), and you will want at least a lb per gallon, even more is better.
A reef tank is obvious. Fish, live rock, inverts, corals, sand, etc.
You will want to invest in some good live rock (at least a lb per gallon, but you'll be better off with more), powerheads for plenty of water flow, lights (the wattage and price will depend on what corals you want to keep), sand (enough to make at least a 3 inch deep sand bed), a skimmer (optional, but will help out a lot).
You can't come to a conclusion about price until you decide which one you'd like to do. All have benefits and downfalls.
Fish only is a lot less costly, but you are limited in what you can stock. Nitrates build faster in a fish only tank since all you have is a filter with nitrifying bacteria, just like in a FW tank. You'll have to either stock lightly, do water changes a lot to keep nitrates in line, or keep fish that aren't sensitive to nitrates.
A fowlr tank will have a lot more natural stuff to look at. Nitrates should be kept in line a bit better if you have a high amount of good liverock and a good sand bed, and if you decide to keep fish that don't predate inverts there are several you can keep. On the other hand you'll have to invest in the rock and a bit higher lighting.
A reef tank will be a much higher investment than either of the previous types. It will need much higher lighting, which is expensive depending on the type of corals you want. Water quality needs to be monitored more closely, and you risk losing a lot more money if you screw up and start killing stuff.
On the other hand they're much more interesting to look at.
Honestly on a 20g or smaller lighting need not be outrageously priced if you decide to do low light corals. There are TONS of low light corals that are beautiful to look at and easy to keep. I wouldn't recommend going with under a 20g though as it will be harder to maintain, especially if you decide to do a reef. A smaller tank will also heat up faster from all the equipment (powerheads and lights put off more heat than you would think), and if you don't keep your house cold you may have trouble keeping the temperature on the tank down.
I'm sure I'm missing a whole ton of information 'cause I can't remember everything offhand, but those are the basics. I've not been doing SW long, so I don't have the depth of info that some people have, but I do have a fresh knowledge of the cost of a lot of equipment needed and stuff.
Decide which one you want to do and then you can decide on cost.
Hope this helps.