You are looking for a helmet with an SA2005 rating, not an M motorcycle rating. They are quite different in construction. The intent is that an SA helmet will protect your head from the multiple impacts with the interior of the car that result from and automotive crash. An M helmet is designed to withstand a single large blow that is typical of a motorcycle crash.
I was able to find pretty nice Pyrotec helmets with the SA rating for just over $100 by searching the web. I don't think Arai makes SA helmets. I know Simson and Bell do but big $$ for the occaisional user. Then again, what is your brain worth?