Don't most BMX racers wear motocross helmets? I'm not sure if there are differences, although my instinct would think that a motocross helmet would be stronger... I think the big things to look out for are weight, ventilation, and safety. I don't know a ton about the differences between a $100 helmet and a $400 helmet, but I'd opt for something that is DOT and possibly Snell certified by one of the major brands like HJC, Arai, Bell, Shoei, etc.
1976 FJ40 - green and mostly stock
1996 FZJ80 - not so stock