Uncertainty and variability are two main specifications of wind generation and the ability of the power system to overcome these challenges is called flexibility. The flexibility index is a measure to evaluate the flexibility level of the power system. This index should show a good view of the ability of the power system and also be easily converted to an equivalent cost to be combined with the operation cost function. Through this approach, a good tradeoff between generation cost and needed flexibility can be carried out. In this paper, a conceptual flexibility index is introduced suitably for real-time operation in the presence of wind farms. Two main components of this index are up and down generation constraints and ramp rate limitations of each unit. Thus, by a combination of the flexibility indices of all units, the system flexibility index is introduced which can be used as the main part of flexibility cost in the objective function. Economic dispatch simulation is used for the economic tradeoff between the generation cost and the cost of flexibility to get the best level of system flexibility in the presence of wind farms considering unit constraints and system loss.