i've shattered two helmets over the years. both times i got concussed. a helmet will NOT stop your brain from ping ponging around in your skull. and that's what damages your brain. it WILL help in preventing your head from getting split open.
While it's true helmets don't stop concussions, the foam in them is actually made with that in mind - it compresses with impact and essentially spreads the impact out over a longer period of time = less damage.
I don't think it's possible to buy a new helmet for $10, I'm pretty sure there are standards helmet manufacturers have to live up to so that you can always count on getting a baseline quality - even if the helmet is cheap. TBH I think there is a lot of "hype pricing" involved with the more expensive ones, it's not that they're necessarily structurally more sound. I payed a lot for mine, but that's because it has a nice liner with speakers.
PowderHound and TreeNinja