Do you really only want to buy race-proven helmets? Or any
racing full-face will do?
I suppose race-proven helmets will be replicas, which will cost you
quite a bit. For daily commuting, a normal racing (not replica)
helmet will do. You can get a standard one like Nolan or AGV for
slightly above $150. I think the cheapest Shoei or Arai cost more
than $300.