Good place for cheap helmets is
www.saferacer.com... I got mine on clearance for around $90 IIRC. You can use SA2000 still, I think through the end of 2010, then they expire. I'm not sure how many SA2000s are still around for sale, as they were putting them on sale like crazy in 2007 to make room for SA2005 ones. But it could save you some money, especially if you're not sure about your commitment level.
Lemme know if you come up to or plan to etown!