A decent Snell SA2000 (car) helmet will run you about $300-400. A really nice one can easily run $650+. Car helmets are VERY different from motorcycle helmets, which are rated with an M-number.
The Snell website has tons of technical details, but the big one is the interior. A motorcycle helmet has very stiff foam designed to keep your head in it's original shape during a single hard impact (e.g. you flying over the handlebars into that f#cking minivan that just ran the redlight). A car helmet has very soft padding designed to keep you from getting too banged up as your head clunks around inside the car during an off-road excursion (possibly involving some tumbling, etc). Also the placement and shape of the pads is different, and the testing & certification process is completely different.
Surely ANY helmet is better than NO helmet, but they are very different beasts...