Depending on the event rules, the M may be just fine. M's are fine for autocross, and some "high-performance driving clinics" where you are on track by yourself.
I've found some really good deals on E-bay. Don't get the used stuff from the private parties, but there are plenty of retailers selling off last year's "style." Brand new helmets. I got an M95 AGV for $65.
Otherwise, G-Force is about the best, no-frills SA helmet value out there. Try solotime.com, racerpartswholesale.com or pegasus.com for starters.
