I have found that fuel costs are not that bad, unless flying mostly larger planes such as jumbo jets (747's, DC-10's, L10-11's, etc...), and if your plane variety is mostly these jumbo jets. So, don't get to discouraged when starting up an airline, and the jet/turboprop you wish to use has a medium-high fuel cost. The plane will be plenty profitable, assuming you fly enough flights with it.