There are no real profit margins to work with in aviation, the numbers are very tight. If you are willing to work with those and realize you will never get rich, it is very doable. I currently run a very small flight club. I also ran some numbers on starting an airport shuttle from my town to KDEN. The numbers don't even compare, the airport shuttle is a much better business.
If you are willing to take very large risks such as above you might do a little better. Also as you move up into the higher end (Selling a lot of Jet A to rich people on their private jets) where service can really differentiate you, the margins seem to be a little better. Read through this board, most of the advice is find the cheapest training you can.
-Jason
www.flyboulder.com