If you do it in cruise you will see very little in the way of cost/fuel management when switching between CI 10 and CI 30 - 40. For these CI's, the optimum level does not change, and your cruise mach number only decreases significantly in the event of strong tailwinds otherwise there is only a couple of minutes in it.
What you will find though is that CI makes a considerable difference to ECON climb and ECON Des speeds. What you will also find is that any CI over 60 - 80 will only cause a decrease in you Opt level once ECON cruise mach is .80.
Just remember CI 0 it the stated minimal ECON fuel burn i.e. LRC, while descent is similar. The saving are such that a CI 10 climb speed will save 20 - 35 kgs of fuel on the climb to the optimum level as opposed to CI 30. The same can be said for descent. If you descend earlier i.e. at a lower speed, your TOD is sooner. Now if your burning 38 kg/min in cruise, and 10 - 14 kg/min in descent, a 1 min delay in your TOD theoretically will cost you an extra 24 kg in Fuel, realistically this is closer to 15 - 20 kg's as you can add an extra minute to your flight time from TOD.
So to this end, actual difference in fuel burn is between 35 kgs and 55 kgs which is why you wont find a change in you FOD on all occasions as the decimal point is not rounded.