helmets you usually make a silicone mold then produce the helmets out of some sort of resin.
armor is usually vacuum formed plastic, pulled over a positive that you make out of something durable (plaster, wood, etc...)
in addition to the numbers above, there's a cost for resin too... around 20 - 25 US a helmet.
it's a LOT cheaper to buy a helmet than scratch build / mold / cast. the latter only makes sense when you plan to make a bunch of helmets.