I can't speak for off-roaders, but for road cyclists and racers I can only emphasise the importance of an undamged helmet.

I'm with Susan, Bikeless, Aint Doody... replace your helmet after a crash... if you end up in an emergency room here, I think ACC will subsidise a new helmet.

If you want to race, your helmet as with all your other gear will be inspected here, and if it has a ding, or even the plastic shell is buckled from sun heat, you are not allowed to ride.

We (my cycling family) all use cheapy $40 jobs from the LBS (except for partners TT helmet)... essentially, helmets are pretty much the same, polystyrene with fibreglass/plastic cover. Short of wearing a motorcycle helmet, a more expensive helmet (from what I have seen, and I invite someone informing me otherwise if I have it completely wrong) only gives you a brand name and a better choice in colour or patterns but very little else in terms of more protection.