I think it really depends where in LA. We live in North LA County (Santa Clarita) and 3 days/week, 4 hours per day, costs us about $475/month (about $10/hour that our child is in school). That's about the going rate at the 3 most popular preschools in town.
There are definitely cheaper ones as well, even as low as $200/month for 3.5 hours a day, 2 days a week.
If you're just looking for playgroups or playdates, join your local chapter of the moms club (
International MOMS Club). There's also gymboree, my gym, and other "mommy and me" classes (try looking at your local community center/parks and recreation guide).