Bump helmets are lightweight, non-ballistic, tactical headgear meant to protect from blunt force trauma and impact. They are usually used in military training or extreme sports.
Yes. It is compleatly legal for law abiding civilians to buy, own, and wear ballistic helmets. Some state may forbid ownership for convicted felons of violent crimes.
Depending on the type and purpose, the price of a helmet may vary greatly. Modern ballistic helmets will cost several hundred dollars even for the basic models.