In reply to Kozma:
ebay is your friend.
Look for a traditional climbing helmet being sold by an outdoor centre, club etc. What happens is that at 10 years old (pretty much arbitrary number) they are considered by insurance companies and health and safety at work etc to be uncertifiable regardless of their actual condition so they must be replaced by outdoor centres etc and they get sold off.
EDIT: usually they bought a batch in the past and so they are selling a batch of them when this happens, so in this scenario the same seller likely selling several.
Got myself an ecrin roc helmet (for some underground adventuring) this way, at a knock down price
EDIT2: *Don't" buy a new one especially not a suspiciously cheap new Chinese (or brand you've never heard of) one, your head is valuable and should have a reputable helmet on it. Look for things like Petzl ecrin roc, Camp rockstar, Edelerid etc
£15 and it looks like an outdoor centre (note the helmet is numbered)
try searching ebay for 133922357644
Post edited at 17:39