Isn't the ATR42 pretty much the same cost-wise as the ATR72 except for the second flight attendant ? (hence why the ATR42 is only selling small number for niche applications these days). Of course capital costs are another issue if the choice is between a knackered- written-down 1990 vintage ATR42-300 and a factory-fresh ATR72-600, but generally speaking the operating costs should not be that different - with the ATR72 giving the airline additional flexibility to cover peak-time demand.