How much it will cost will be dependent upon many factors. New or used equipment, how much DIY you can get, if you get local deals from local reefers, what you stock with, how prepared you are, etc.
I can tell you I've bought a lot of stuff used and I'm not nearly done and stopped counting at about $1000. I'm only 3 months in the hobby. I have a 55. I've heard of people spending that much on nano cubes. I'll be spending much more on mine.
When you say you want a SW fish tank. What kind of fish appeal to you? Do you want inverts like starfish, shrimp, crabs? How about corals?
The list of what you need is so dependent on what you want to keep I might scare you with what I'd start off with and you don't need it. For instance, I have (2) 250watt Metal Halides over my 55g. That is going to be expensive if new, and you might only need basic lighting which will be far less in cost.
Another option, if you have a budget in mind, give us that and we can help with how we'd best make use of it to get started.