The base fare of rajdhani/Shatabdi/Duronto is already higher than the ME base fare by 20% and in addition to it SF/Reservation charges are also levied. So they are already costlier than the SF trains bu 20% of base fare.
It is the journey time which matters to passenger and not the max.speed. If extra fare is to be collected in the name of speed, the best solution is to create a new class of special superfasts with avg. speed above 65/75 kmph and keep fares accordingly higher.