Hi F,
Cost index is an input into the Flight Management Computer that balances cost of fuel against cost of time. Time includes crew costs, maintainance costs and all other costs associated with hull ownership. It is simply a ratio of cost of fuel over other costs. Therefore a cost index of zero input into the FMC will give the least fuel required for a flight. At this point it is necessary to realise that the resulting speed demand will be dependant upon weight and wind inputs - real time wind for a while and flight planned winds for projections. To minimise flight time regardless of fuel usage on Boeing aircraft with Honeywell FMCs a maximum cost index of 999 could be utilised. I'm not sure of the Airbus equivalent figure. The average cost index in commercial use is about 90 but it does vary depending on the airline and their view of the balance of costs, including timetable robustness.