It depends on a lot of things. The skimmer, lights, sump, pumps, stand, etc. On top of that, it depends the quality of each of the equipment or whether you want everything brand new or used, DIY or ready to run. On DUY depends on what things you use for example the wood you use on the stand.
I would suggest you start to research before to think about cost. You could say about over a thousand dollars if everything is brand new which I highly don't reccomend since you could find used equipment for less than half the price and sometimes better quality than the one you're thinking about purchasing.
I got my 75 gallon in December and have put at least $2000 in it so far. I highly recommend that you don't buy a tank online. Whatever you save in cost will bite you in shipping. Not to mention the hassle if they ship it and it has a leak or something.