I own three stores. One has (had) fish. I built/opened it 6 years ago, and with the current economy being as down as it is I am shutting it down and moving to a smaller spot and dropping fish entirely, in fact all live animals.
My store with fish is 3000 square feet, I have 120 tanks give or take, 20H's, 40L's, 40BR's, 65's, 125 and 180. About 6000 gallons of water with all filtration sumps and an indoor pond. It cost me 30-35K to build the fish room 6 years ago. We built everything ourselves from the sumps, to all the electrical, the racks, the plumbing, etc. we did everything, no contractors. Also I used a lot of free, salvage, 2x4's to save money. I drilled all of my own tanks and researched what pumps to buy, what lights to use, what heaters, what filters, how to plumb efficiently (what size pipes), we built the sumps out of plywood, 2x4, and pond liner. We faced everything in Oak and made doors to hide all of the plumbing, electrical, etc. All of the wood was painted in oil based paint...... Anyway, you can do it, but you have to be willing AND able to work non-stop for years and do whatever it takes for your baby (ahhh store).
The short of it is, you can't just ask someone what it will cost, YOU have to figure it out yourself, because when you do your business you are on your own and you have to watch every penny.
So as someone else posted, crunch all your numbers, do your best estimates, and then DOUBLE or better yet TRIPLE it!!!
If you have any specific questions I will be happy to help. Ohh yeah, the bad part, you have to do everything in your power to not let it, the store, the customers, the wholesalers, suck the love of the hobby out of you, b/c there will be times where you ask yourself, "do I even like fish?"