This is the same question as "How long is a piece of string".
The answer, of course, is "that depends". A lot of people are cagey about how much they spend, mainly I think because it's so damn expensive, but I think it's better to have that information out there than not, even if (right now, I may feel differently in a year or so

) I'd rather not have spent that much cash...
Basically to answer your question, you need to get a lot more specific. You can get standard tanks (MarineLand 300DD for example) that are large, and they'll run about $2400. Or you can go to a custom tank manufacturer (AGE, Miracles in Glass, ...) and spend anything from $5k or so upwards. The sky really is the limit here.
As an example, I'm getting a new ~400G tank from Miracles - the tank cost for 84" x 36" x ~32" in starfire glass all round (the exact shape is oddball - see
http://reefcentral.com/forums/showthread.php?t=2454671 if you want to see the end result and find out why I wanted the back in StarFire) was $7021. I don't know if this is typical, but it was the mid-way bid from several manufacturers for my particular requirements.
BUT, then you have to get the stand as well, and I wanted the same people to make the stand that made the tank - that way there's no finger-pointing down the road if there are issues. That's an extra $1065 because I wanted removable center-braces so I could slide the sump in. Oh yeah, the sump was another $600...
THEN you have to actually get the tank to your door. For me, the shipping came to $2250.
So, about $11k to get a
new tank/sump/stand delivered (at least for me. YMMV) - that doesn't
begin to describe the costs though, there's the rock and sand, there's any foundation support (yes, I'm doing that for the house) and there's the appliances that support the ecosystem (lighting, skimmer, plumbing, various reactors). Don't forget any permits (a building permit cost me $1200 or so) if you're making any structural modifications to the house.. It adds up pretty quickly, especially if you're starting from scratch.
You can save a
lot of money going used. Tanks are way worse than cars for keeping their value, at least for the first 50% or so, after that they're pretty stable...
As a rough guide, my budget for what I'm doing in the above link is about $30k. I am pretty much starting over though - new Bubble King skimmer, a house renovation to install the tank, etc. etc.) Instead of getting a new car, I'm going to stick with the old one for another 5 years until it's a decade old, and get the fishtank installed instead. Everyone has their own tradeoffs, and I have a very understanding wife who's just lost a chunk of her living room
Simon