Depends on if you want to be a good lfs or one of the stereotypical lfs that provide bad stock and bad education. I'm hoping you are wanting the former.
If that is so, then you need to consider a few factors to estimate cost. First, what size store are you looking for? Is there a suitable space available for rent or would you have to have it custom built? How much is commercial rent in your area? How much is power? How much is water? Insurance? Those are going to be your biggest long-term costs, so you want to make sure you can handle them. Most businesses don't start making profit for a long time (2 yrs +), so plan accordingly.
After you've figured out the location issues, you then need to figure out how much stock you are going to be after. Are you going to be only a fish store, or are you going to sell other animals (reptiles, birds, etc). How many sales tanks are you going to have? How big are they going to be? Will you have any display tanks in the store? Are you going to be fresh and saltwater? How many quarantine tanks are you going to have? Are you going to do any propogation in the store (i.e. fragging/breeding).
You also need to find suppliers, both for livestock and equipment. What are the minimum orders the suppliers require? How much is shipping (it can be killer).
The final question is if there is even a market in your area. If there is, how many competitors are there? Are they large chain stores or locally owned stores?
If you want to do it right, I would say at least $150-200k and 6-12 months of planning and set-up. Then you are looking at another 1-2 years minimum before you really start pulling a profit.
If you have the means, the knowledge, and the motivation, go for it. Retail is a tough business though, and pet retail is not the easiest form. However, if you do it right and have some good luck, you might just become one of the "good" lfs that people travel from far and wide to purchase from