If you're going to your first autocross there's a good chance that they have extra helmets you can use. Every autocross event I've gone to has had at least 10+ basic helmets available for people who did not yet have their own.
You should definitely call ahead before you rely on that but if it's available it would give you a chance to see what people use and what would work best for you before you spend a few hundred on a good helmet of your own.
Also, the M2010 & SA2010 helmets will be out soon (heard that simpson's were coming out this month). New ratings come out every 5 years (2000, 2005, 2010, etc) and most places only allow you to use the two most current ratings (currently 2000 and 2005, soon to be 2005 and 2010).
If you pick up a 2005 rated helmet it will only be usable for 5 years but if you wait a few weeks for a 2010 it'll be good for twice as long. The M & SA 2010 helmets will probably cost a bit more though so only go that route if you plan on using it for a while.