Look, I would wear a helmet (full face) whether there were a law or not. Still, I hate to regulate any adult (kids are different) without overwhelming reason. I’m not sure insurance cost does it for me. Would you mandate a certain diet because health insurance costs would be lower? How about forbidding any risk. Hell, motorcycles are more dangerous than cars, why not ban them? Freedom is precious, be careful how you curtail it.
As a practical matter, do helmet laws decrease insurance costs? It seems like a dead person costs less than saving a live one. I remember a news article about a consulting group that did a study for a country in eastern Europe. As I recall, they cited smoking for shortening lives and reducing expenditures on care for older citizens. As far as I know, the recommendation was not implemented.