To give you a quick breakdown for price on a 30g tank.
Corals require light and light requires money. You can get away with T5 and maybe even PC
lighting on a 30g tank if you are going to house low light corals. I've seen 30g setups with computer controllers and 250w of metal halide lighting. The sky really is the limit when it comes to these systems.
What you will need:
Lighting: probably cost between $150 and $350
Skimmer: get a good one! $150 - $300 (depending on brand)
Live rock: 50-75lbs @ $3.99-$8.99 per lb
Flow: corals require flow 20-30x turn over per hour (30g tank = 600phg - 900gph) This is just the flow in the tank you can achieve this with power heads
Filtration: I don't recommend HOB
power filters or canister filters for a reef tank. I suggest the above amount of LR
and a good skimmer along with a refugium (google it)
A Sump is a very good idea which means you will need an overflow box.
The point here is having corals is going to cost you a LOT more then just having fish. Corals themselves are very expensive specially if you are looking for those really interesting beautiful pieces.
You could easily spend well over $2000 on a 30g setup.