Well, this isn't exactly anything new under the sun, but I was thinking about this during my drive last weekend. Boat manufacturers, please take note.
I hate trying to calculate what each person should contribute for a set, especially given the differences in time that people ride. So... the first manufacturer to include a fuel usage meter in their newfangled dash displays will get a strong kudos from me. It would be fairly straightforward to put a fuel meter in the fuel line (or take injector duration x flow rate) and then display the cumulative fuel use for each rider on the dash display. The manufacturer could even have inputs of cost per gallon and % overhead to present a cost per set that is customized for each rider. Then I would know how much to charge each person, without having to subjectively guesstimate.
Just an idea...