Do you have a set of the stock 17x8 BBS wheels for the car already? They're pretty good wheels for stock-class autocross... fairly light, really strong, and for sure they meet the rules. If you just need another set, Emily is selling her stock gold wheels (here:
http://www.seccs.org/forums/showthread.php?t=6644) for so cheap it's insane.
Or you might be able to save some money if you race on the your current stock wheels and just pick up some beater wheels for cheap to put street tires on to get to/from the track. Hell, I've got a set of 17x8 +48(?) A-Tech Erasors I'd sell ya for $200, but they're pretty heavy for autocross compared to the stock wheels.
Another option is springing for a set of 17x8 +45 Enkei RPF1's... which are about the lightest wheels you'll find without going to ridiculously expensive forged wheels. Edge Racing has 'em for a little over $1000 for a set:
http://www.edgeracing.com/wheel/6081/ But I'm not sure if +45 offset is legal... aren't the stock rules really strict about that stuff?