A big part of the DOC is the price of fuel. In the states it can range from $4.20 to 5.80 USD per gallon. If you plan to go Part 135 (air taxi) the insurance could be from $15,000 to $35,000 a year. I hope this doesn't scare you away, you have to charge enough to cover all costs and to make a profit.
I have set up a 135 operation with an amphib C172-180hp.