It can be as expensive as you want it!
Since your asking i'm assuming you are on a budget so here is a rough list:
You should be able to reuse tank, heater, and test kits(assuming you have the saltwater cards)
Rock: $102 On a budget get a good dry rock rather then live I like marcorocks.com. Get about 25 lbs of dry(62 from marcos) then go to your lfs
for 5-10 lbs of live(price will vary but should be around $6 per pound)
Lighting: $180 You said you wanted corals so I would look at one of the 120w Chinese value fixtures. Look to reefbreeders or TaoTronics on ebay.
Powerhead: $45 I'd do 2 of the Koralia nano 425s
unit: Around $100. This isn't needed but unless you want to lug water from your LFS
is makes things a lot easier. DO NOT USE TAP A lot like the Bulkreefsupply units if you want to save a bit there are decent options on ebay for a little cheaper.
Skimmer: $100. This is also not needed but is helpful to remove waste from the water. Can be added down the line also. Several options for a tank your size.
Substrate: Couple bags of argonite $25
Salt: A decent salt mix should be around $50 for bucket or box.
Added up for everything it is about $600