http://www.bobscycle.com/
This is where a lot of the autocross guys go, and where I got mine. The selection is massive and you can get a basic helmet for very cheap. One guy said he got a full faced snell 2005 for under $100.
Open faced snell helmets from $60 on up.
Oh, and they are local. St Paul just off I94. So if you are in the area you can just go in person and save on shipping, plus you know what the snell rating is. When I was there I think he said all of his helmets were Snell2005 but I wouldnt order online without verifying that.