3-4 times more effective than fitting the inside of the car with padding. Up to $600 AUD per annum decrease in accident associated costs in 1997 dollars. http://www.monash.edu.au/miri/research/reports/atsb160.pdf

That seems well worth while. Interestingly I have to wear a helmet when I walk (when my employer may get sued if I'm injured). I'm required by law to wear a helmet on a bicycle, motorcycle or hang glider. I have worn helmets to swim, climb, fly a plane and ride a horse. About the only transport situation I've never worn a helmet is in a car. I think that must be some sort of cognitive hole.