It would seem to be a good idea to place an armed guard on board the truck. Would it cost money ? Yes.
Would it cost 30 million dollars a month?? I doubt it.
There is someone already on board the truck, why not arm him? I think the value of trailer loads is approaching that of Brinks trucks. A $40-50,000 a year driver is not going to sacrifice his life (nor should he) for a load of consumer goods.