For an autocross helmet just go to the local motorcycle shop. The HJC brand of helmets is the most popular helmet at the SCCA Solo Nationals and a couple of winters ago I bought a HJC CS-5 (open face) at the local Honda dealer for $65. I like it better than my Bell Mag 4 that cost twice as much. Just be aware that not all helmets sold at a motorcycle shop will be Snell approved (needed for SCCA Solo competition). Make sure it has the Snell sticker inside on the foam before you buy.
OBTW- Many SCCA Solo National Championships have been won by folks wearing a <$100 helmet.