Cost: You'll need a helmet and a $20-$50 entry fee, plus SCCA membership dues ($65-90 depending on the region) if the event is sanctioned by that body. Don't forget to factor in the wear and tear of having the underside of your car assaulted by gravel too.
Race Format: It's autocross on dirt; nothing but cones and an open field. Whoever sets the astest times wins.
Risk of Bodily Harm: Like autocross, it's very difficult to get hurt rallycrossing. If you've always wanted to rally race but really don't feel like dying by driving off a cliff and impaling one's self on a tree, this is perfect.