Helmets range from pure junk (think china knock-offs for $75) to high end, advanced protection (Shoei, Arai, 6d and some others). At the end of the day, you get what you pay for. I guess it all comes down to what you think your head is worth. More important than ratings (Snell, DOT, etc) which may or may not apply to off-road use, look for features like removable padding (cleaning & replacement), MIPS, custom cheek pads for a better fit, etc.
Three items I'd never skimp on: Helmets, knee protection and boots. They can be expensive, but surgery costs a lot more and recovery sucks.