To reinforce what Calesta said, be sure to check your sports governing body for their regulations.
Most manufacturer and car club track days support an M2000 helmet, which is what I just picked up for $80. I got an HJC with visor, basic package. CART and FCSCC both support it for track days, and of course autocross. DGTA allows M95 or Snell95, their motto "Hey, it's your head".
Any open wheeling is going to require full face shield, and most driver's schools do as well (Bondurant as well as Skip Barber require full face, but Petty does not)
I bought a used helmet last year, a Snell95 HJC full face. The helmet was good, but last weekend someone just dropped their bike, took a slight bang to the helmet and it cracked. So buy NEW, and take very good care of it. Get a soft helmet bag, or something with a hard inner floor to protect it. my HJC was dropped, maybe twice.
If you're biking, lightweight may be an issue for you. If so, then look at the Kevlar / Carbon helmets. They start at $160 from HJC.
-> Steve
If you're in Connecticut, I have the number of a privateer that I bought mine from. He doesn't do internet offers.