Monte Carlo simulation code using matlab

I am trying to run stochastic optimization using genetic algorithm command from the built-in command available in Matlab. I have succeeded to run a deterministic optimization using "ga" command. I want to convert my problem to be a stochastic optimization problem by optimizing the expected value of the objective function using the same algorithm (genetic algorithm). I wonder if you can help me convert my problem from deterministic to stochastic. I am trying to maximize the Benefit/Cost ration of maintenance and rehabilitation. the benefit is estimated by evaluating the area bounded by the function and the threshold values and the costs are estimated by using different component of costs.