There really isn't any way to estimate a cost other than to say, expensive. If money is not an issue for you, then great, wonderful, you could do a full blown reef in a 55 gallon tank without a problem. If your budget is severely limited, then you might have problems. Well, I wouldn't call them problems really, it'd be more like well, putting off that dream reef until you can afford the necessary equipment.
Reefs are hefty investments. I can't stress enough RESEARCH RESEARCH RESEARCH. Do your foot-work and go to a good bookstore or library. There is literally -tons- of information out there about how to set up a reef, and by looking up your equipment needs and making a shopping list, not only do you know right from the get-go what is and is not appropriate (because most LFS employees won't), but you can price items at differant places as well and buy the right equipment right from the start.
Some people start off really cheap (I had a customer do a fish only SW 55 gallon for $700 on an impulse buy because he wanted a SW tank, and I set him up as cheaply as I possibly could). Then they add on other stuff as they learn more about how to properly take care of their tank (same customer went out three months later after doing some research and bought a $150 UV light, then went and spent $60 on a skimmer etc). Thus far I'd have to say this customer spent at least $100-200 on fish alone in my store, I don't know if he got other things anywhere else.
That 55 fish only tank, after two years or so, has cost my customer at least $2000 with everything he's added, including fish. A reef tank is going to cost a bit more because inverts and coral are -expensive- and so are the lights -required- for their thriving.
And please, know your critters. Inverts are really really delicate, espeically corals and anemoes. They look really cool, but as any reefer could tell ya, you have to have your heart in the animal for it to thrive. If you just want it to look pretty without the necessary time investment to take care of it, you'll have dead corals in no time. RESEARCH RESEARCH RESEARCH.
~~Colesea