From the Airbus FCOM Vol 1
COST INDEX (CI)
CI is the ratio of flight time cost (CT) to fuel cost (CF).
CI = CT/CF KG/MIN or 100 LB/H
The cost index is used to compute the best economic speed and Mach to be flown considering the ratio between the flight time cost and the fuel cost.
CI = 0 Corresponds to minimum fuel consumption (max range).
CI = 999 Corresponds to minimum time.
Rgds