It can really vary. Anywhere from $3,000 on up to well over $10k, even more.
My 180g is the size you mention, and I'm not even sure what the total was. I would have to estimate it was around $8k, and that's probably somewhat midgrade, and that's just the tank. Never done the math, let me see quick.
I bought a prostar 230. So that was tank, stand, sump and plumbing done. I paid $2700, today they sell for over $4k.
3 Kessil a360we. I got them on sale for $300 each, normally about $350. So $900. Also another $237 for 3 mounting arms, and another $60 for the 90degree piece. So basically, $1200 for lighting.
For powerheads, I only have 2, many people get more. But I picked up 2 MP40's. $350 each, $700 for powerheads.
Protein Skimmer, $350 for the one I have.
Refugium light: Another kessil light, $240. I already had a cheaper mounting arm, but I think they are like $45. So $300ish.
Return Pump: I went cheap here and got a Jabeo pump. $90.
Kessil Light Controller: I already had, but $100.
2 300w heaters: $50
InkBird: $35
UV pump: $60
UV Light: $250
Live Rock: I have negative space aquaspace, and only used 100lbs. I bought the cheapest, so $350.
Marco Cement + instaset + superglue: $90.
Salt - $60
RODI Unit from BRS: $350
Sand: $120
So, I think that's the equipment on the tank. That comes out to $6,805 in equipment.
Then there is all the fish. For me this was very expensive due to losing almost all of them once to velvet. Probably about $900 I spent total in fish.
$7,705.
When it comes to buying coral, I only buy coral on sale and I buy the cheaper ones. I spent about $600 on corals, but that's the really low end. I already had a bunch of coral already as well.
$8305
As mentioned, I had a bunch of fish die. So I had to buy a bunch of stuff for quarantine. Fish medications, multiple tanks, filters, air stones etc. I'd estimate that was about $400.
$8705
And then you need testing stuff. I have 4 hanna tests, lets say $50 each. Then I have some salifert tests, probably $100 total. So $300 for tests, but I already had some of them.
$9005
I'm probably forgetting something, but yeah about $9k. I am very pleased with the end result, but I had no plans to spend that much when I started.
Oh yeah, dosing pumps, dosing containers, etc. Probably $100. Then there is the dosing solutions, power strips, $200. Clean up crew, $200,
And then if you want to use an apex and things like that, you're adding $1500. Maybe someday I will have one, not today.
Take whatever your budget is, and cut it in half when you plan, you'll likely spent the full budget that way. Or find what you want, and double the price.