I noticed cycle helmets in-store priced at £2.99 for children and £3.99 for adults. I did not notice any make on them (I'll have a closer look next time I'm in and will try to get a photo too). UPDATE - The make on the helmets is Star Sport.
They will not be top quality at this price, I suppose, but they must have passed safety checks.
I reckon you can't go wrong at those prices. What do you think?