Hey,
Just for a reality check I think it would make sense to review the concept of grass taxiways and service roads.
While they are cheaper and good for beginning, I believe they should not be usable for heavy trucks/buses and medium aircrafts (have no game access now but I believe it was possible to taxi on grass with medium a/c).
Maybe worth to review pricing by the way, as grass would be less usable.
Thanks, k.