I would be very interested to hear if wearing neck collars would reduce injuries in heli accidents and have considered wearing one myself after F1 introduced the HANS system, where a rigid support is worn over the shoulders to which the helmet is strapped.
HANS wouldn't work in a heli as I think it would restrict head movement too much.
But, priced at under $30 the basic foam wrap around collar types aren't expensive and the racing car driver collars come covered in fire resistant materials.
Neck Collars
Pros and cons of wearing one?
Mickjoebill