With 8 in a vehicle box, that means the price is still $5 per.
If you will forgive me indulging in a bit of "Reductio ad absurdum" (i.e. showing an absurdity to which it leads when carried to its logical extreme conclusion), would you expect to pay eight times the price of an individual Atlas for a Vehicle Box containing 8 Savannah Masters??
Obviously picked the example of the biggest mech I could think of, and the smallest Vehicle I could think of, off the top of my head, to show why I personally don't think Vehicles should be $5 per vehicle.
The point that others have raised earlier is that the typical/average volume of a mech is larger than the typical/average volumne of a vehicle, stating volume to allow for height, width and length. Is an "average" Mech twice the volume of an "average" Vehicle?? Maybe not.
So I am trying to say that a Vehicle pack (often seemingly with 4 larger and 4 smaller vehicles, on average) should cost less than the equivalent of 8 Mechs but probably cost more than the equivalent of 4 Mechs. Hence why I mentioned the price of an 8 Vehcile Box being similar to a 5 Mech Star or a 6 Mech Level II.
That in my personal opinion is around the right price for a 8 Vehicle Pack.
My opinion, your's or even more pertinently, Catalyst's, may differ.