Most of the upgraded ammo’s prices make sense to me, because they’re roughly the same quantity but just of more expensive materials. Some cases such as the mortar and gatling gun are arguably cheaper than their hand grenade and SMG counterparts. The high-explosive mortars, though, are so absurdly expensive that I can’t imagine ever using them. The game just doesn’t have any stand-out, high-difficulty bosses that would warrant dumping an obscene amount of resources into any advantage I can get.
If the price is because of how powerful they are, maybe there just shouldn’t be a x2.5 damage jump in effectiveness? I don’t think there’s any other weapons with such a drastic increase. A x1.66 jump would put its damage per second roughly on a line with hand grenades, which seems pretty fair considering the HE mortars are T3 weapons and hand grenades are T1.