what ur forgetting is inf still use trucks to tow their arty aa and a/t. with bigger and better support weapons the bigger the truck hence more fuel and supplies used. also a higher ic cost to build. armour aircraft and ships will also use more cause they also get bigger and faster. tanks also get heavier as u add better guns and armour. so all costs will go up. the cost in the game isnt as high as it was in the real war. u also have to take into account the extra supplies and fuel needed to get the stuff to the units. even the parts to upgrade units have to get to the unit which means more fuel for that.