UA segment runs in CA: eg, SMF-SAN $37 one way
WN is having a heck of a fare sale on their CA routes, with $29 one way fares. UA is actually matching these and the beauty is that most of them force a connection in SFO or LAX when traveling between secondary markets so they are quite lucrative for segment runners.
Here are two examples:
SMF-SAN for $37 each way via SFO or LAX
SMF-BUR/SNA for $37 each way via SFO...for whatever reason ONT isn't included.
$18.50 per segment is pretty good these days. For SAN, you may be able to SDC to a double connection in both SFO and LAX, driving the segment cost down to around $12 per.
Not sure if these are basic economy, I didn't check.
It's a Tues/Weds/Sat fare and availability is scattered until early Feb 2018.