Southern California Equestrian Directory
Also Rancho Del Rio in Anaheim...
Sierra Vista is lovely (as lwg has said above)
I'm at the Costa Mesa Equestrian Center (where they hold the OC fair) and our stalls start at $555 a month. That's all inclusive and you're allowed to jump without a trainer present.
Huntington Equestrian is relatively cheap for pipe stalls... however you have to provide your own shavings, and in this area shavings are overpriced!!
Anything in the LA county area will be less expensive. Paramount, Downey, Long Beach, Cerritos, Los Al, and so on.