The most effective system I've seen is the DL90. It's $700, but it's fairly easy to move from one car to another (install softside velcro on the dash) if there is interest in a "group" acquisition. Data analysis is very quick since all data points are corrected by GPS position at 5Hz.
Alternatively, the GtechPro Competition might finally get a firmware upgrade sometime this year that will permit logging autocross runs... We'll see.