That's a tough question to answer. There are a lot of variable costs involved:
Will you purchase or lease airplanes?
New or used airplanes?
What airport will you work out of?
Will you rent space for offices from the airport?
Hull and Liability Insurance.
Business Premises Insurance.
Who will provide fuel and oil?
Who will provide maintenance and inspections?
Tie down or hanger fees?
Yellow Page ads.
Start up office equipment and supplies.
Weather Service Subscription.
Will you try to obtain business financing?
The FAA does not really get involved in a Part 61 flight school. You can start with a single airplane if you can get the insurance.
You will need to find out what the above costs will be on a monthly basis. You can check out other flight schools in the area to determine what you can charge per hour and how much business you are likely to do. You will then have to determine if you can rent your airplanes enough each month to pay the expenses and throw off a profit. Don't forget to include putting aside money for engine rebuilds, inspections, and unexpected maintenance. You have to market well and control costs in order to be even marginally profitable in the current economy.
It's sad to say, but cheap labor, i.e. desperate flight instructors, is easy to come by. Instructors are usually contractors but you may have to pay worker's comp depending upon the state you are in, and liability insurance.
Profit margins are small and the risks are high, so think carefully before trying a start up flight school.
Some excellent posts giving you guidance on how to start a flight school. My $0.02 worth is ensuring you have a strong market (location), and that the weather is good enough to be able to keep students flying year round (again....location), and finding instructors that will help you build an excellent reputation. Quality is the key in instructing, since we're all selling the exact same product (FAA certificates).
As has already been said, unless you're in it to become the next Flight Safety, profit margins are small, risk is high and it has to be something that you LOVE doing because it takes a lot of hard work, time and money to see it through.
I do not own a flight school, but I have been responsible for the creation and development of our local school, including a 141 operation that is linked to a college.
If I were to start my own flight school tomorrow, I would think about starting a combination flying club / flight school. I was discussing this idea with an area airport manager and he seemed very enthusiastic about the prospects.