Thanks to everyone for your comments.
Harrier46, have you seen any flight planning systems so far that will either plan at the optimum cost index, or fly a longer route which has cheaper nav charges at a higher cost index (to reduce flying time to stay within a given block) to achieve a saving?
I know there is a "Required time of arrival cost index" feature in Jepps, but running that with optimum scenario analysis is beyond the boundaries of jetplan.