How about at The Dirt? They have $5 open practice on sundays from 11am-4pm according to their website. I've never been there, but from what I can see in their gallery, the track is made for 1/8th scale. I know these guys are a popular track so it may be crowded as well.
*edit* HotBox may be a good option as well since its right in between LA and San Diego. The track looks like a 1/10th track though. Or.... we could go to SV Speedway in LA. That's a 15 min drive from where I'll be staying.
Here's a list of tracks ripped from a rctech thread:
Revelation in Ontario
http://www.revrace.com/
HotBox in Santa Ana
http://www.hotboxrc.com/
SV Speedway in Sun Valley/Burbank
http://www.onlinerc.com/
Crystal Park raceway in Compton
http://crystalparkrcraceway.com/
The Dirt in Hemet
http://www.thedirtracing.com/
Hot Rod Hobbies in Saugus
http://www.hotrodhobbies.com/Main_frame.htm
Thunder Alley in Beaumont
http://www.thunderalleyrcraceway.com/