Your plan of action:
Calculate the following:
-price per hour to run 50 systems at peak power usage for however many hours of the day they would be on.
-price per hour to operate the rest of the building's systems for however long they would beon per day.
-desk employee wages.
-security guard wages.
-calculate how much it will cost to pay someone to clean up your machines every time they're due.
Decide how often you will alter each machine. ie;
-re-ghost all machines to a fresh image every morning/midmorning/week/2x a week.
-defragment all machines every morning/midmorning/week/2x a week.
-rotate the defrag and ghosting cycles so there are always 40 computers open for use, and 10 that are being defrag/cleaned.
These are just some getting started areas. I'm sure you've thought of these before, but, I'd just like to reccomend that if you look to calculate a profitable experience at a decent customer price, you're going to have to consider when you're open, how much of the above will be nescessary and how often.
Some friends and I were going to do the same thing you're doing.. of course, that was back when the fastest computer was 700mhz, and it's price was over 400$ just for the cpu. My point is, though, that you should really be positive you want to go through with it and that there is an active demand for it in the community. Also develope promotional ideas to get kids in there instead of out on the streets (market to the parents, but be casual and not authoritative about it.)
You may not be interested in this... but a "Cult Classic" movie theare on the side which shows gamer-known favorites like the Evil Dead series may keep your gamers in your building spending their money at the food counter as they chill to watch a movie in between gaming matches. (I know I and my brothers both take breaks every 3-4 hours to watch a movie, get some food, or just zone out.) A movie projector can entertain many more people for a fraction of the cost that the same number of computers may entertain for the same price. Show action movies too... gamer-geared movies. 2$ for a movie is pretty cheap, especially when it's a movie you know you'll like to see.
Anyhow, before any thing else, calculate the demand for such a thing in your area. I'm not saying you don't already know if there is one, but it's a good idea to make sure there is before you dive in to some thing this big. We decided that for the cost per month of services in our town, the profits would not match the expenses with out charging guests an arm and a leg, even with our orriginal Computers being half-priced through a friend's uncle.
Goodluck and Peace!