David
The answers so far have not been vey helpful, have they. To obtain an AOC is a very expensive and time consuming business and if you put the true cost of doing it your airline would never get off the ground, or if it did it would be burdened with debt before the first passenger paid for a ticket.
For a small start up with 2 or 3 small aircraft (ATR's) the best bet would be to "piggy back" somebody elses AOC. This way you get legal easily and, more importantly, quickly. I would hazard a guess that another operator would provide this service for about £5,000 per aircraft per month. Put this in your spreadsheet and at least you have something to work with. Good luck with your project,. Are you at Loughborough by any chance?