As someone pointed out in this thread, this would discourage carpooling with eg minivans. Coming from a large-ish family of modest means, that would have cut down on the number of “field trips” my mom took us on which would have been a shame. Instead, how about (car volume * size of blind spot in front of car) / (number of seats)?
Why not get the next model of minivan up? Perhaps it didn't justify the cost. Why not get a 2nd hand one? Or one from a different manufacturer? Perhaps the cost was affording the best one deems within the means. That extends to eating out when there vs a packed lunch or picnic, things to bring.
There's always costs, options. Where, and to whom, you're normalised to allocate or offload it to.