Cheaper than just about any other way to get from SF to LA, though having the LA end in Santa Monica kind of casts this as a SF-Silicon Beach commuter rather than a regular service. Also, it looks more like a modern version of the old Pullman sections rather than roomettes, perhaps a class up from the Green Tortoise.

As far as fare is concerned, inflation adjusted it's even cheaper than Pullman sections in the 1950s, and nowhere near roomettes or other "1st class" sleepers. That $48 fare can't last, though, especially if they run legally with fully licensed drivers etc. Neither Amtrak nor the regular bus lines can compete with it. Note that present Amtrak overnights between SF and LA are mostly bus rides.

Closest Amtrak stop to Santa Monica is Westwood, on bus connections from the Surfliner (via Van Nuys) and San Joaquins (via Bakersfield). There are 7 SF-Westwood runs at the Amtrak website, but they're all daytime. If you're willing to spend another hour+ on transit, of course, it's easy to get from LAUS to Santa Monica via Red/Purple (in the LAUS basement) and Expo lines. Main SF Amtrak station is at the temporary Transbay Terminal, shared with Greyhound and Bolt, which isn't far from the sleeper bus stop on the Embarcadero.

Current Amtrak overnight, all use LA Union Station rather than Westwood. SF-LA bus & Surfliner is 2245-0935 with a transfer from bus to train in Santa Barbara at 0630, $59-$109. San Joaquins SF-LA is 1710-0220 with train/bus connections at Emeryville and (at midnight) Bakersfield, $59-109. Northbound Surfliner is 1915-0535 with change to bus in Santa Barbara at 2150, one San Joaquins 0115-1040 with transfer to train in Bakersfield at 0425 and to bus in Emeryville at 1010. The San Joaquin route isn't very attractive with those timings.

If all you want to do is get there overnight, several late Greyhound and Bolt buses run between several LA points (though most seem to serve Union Station) and SF (Greyhound at the Transbay Terminal) for $46-52. Most seem to take around 7.5 hr. Or try Megabus with 2325 & 0015 departures from SF Caltrain arriving at LAUS 0700 & 0800 for $41-48 (and up).

For the record, Southwest's Wanna Get Away (restricted) fare, one-way SFO-LAX, is $174 for short booking, $69 for booking several weeks out. Plus airport transfers, of course. LAX isn't really very far from Santa Monica. But no overnight flights unless the incoming of the one you're booked on is very very late (it happens).