Hi, Tosh!
Welcome to the board!
The problem with your question is that there isn't any one answer! There are many types of saltwater set ups, from a simple small tank with fish only, to a full blown reef aquarium. Cost is pretty much dependant on what you decide you want to do.
A good way to decide is to browse boards like these and look at lots of pics of different sizes and types of saltwater aquariums and make your choice based on what you like!
A beginner can set up a fairly low cost small tank if it is intended for fish only. You will need the tank of course, followed by substrate if desired, live rock, a small power head or two, a small protein skimmer, and hydrometer. A small tank can only support a couple or few fish and you would need to select carefully based on size at maturity, compatability,and the individual species need for space. Good choices for small tanks are typically a pair of clowns, small gobies, dottybacks, royal grammas, and other fish of that type that are relatively small, hardy and non-agressive.
Reef tanks need all of the above equipment plus some type of high powered lighting. power compacts and vho's are a good choice for small tanks if you don't intend to keep sps corals or clams (though some people do, successfully). Metal halide is the typical choice for a sps/clam tank.
Cost can go up very, very quickly with reef hardware, you can pretty much imagine any amount and then double that
.
Skimmers are not exactly necessary, but being a reefer who has run tanks with and without them, I now consider them indispensable. A smaller hang on the back type is usually sufficient for tanks smaller than 55 gallons or so, though this is an area that is hotly debated.
Alot of times if you save and wait you can find used equipment for a fraction of the price, even whole set ups occasionally.
I'm not sure of where you are located, or what the laws are in Australia, but having the GBR there could definately be a huge plus!!! Check before you collect, but if there are open areas you could definately save alot going that route.
Have fun and good luck!!!
-Becca