One way car rentals are very expensive. I travel frequently, and if you have a Costco membership, they have the best pricing.
Mostly true, but not always. I usually book on Costco and then run my numbers through Autoslash. About 4/10 times, they will return with a MUCH better rate via Priceline (or their own internal booking) to one of the majors. The other 2/10 it's usually an off-site brand that I'm not familiar with, the rest Costco wins. I find that as the time approaches (2-4 weeks before travel time) prices trend down absent a major event.
OP - I did a random search in March for 9 days (5-14, pickup LAS, dropoff SNA). $600 on Costco, $594 on Autoslash....I'm thinking it's way too early to book.
Have you thought about flying? You can rent a car for 3 days in Las Vegas for $110, fly on JetBlue from LAS-LGB for $208, and then rent the remaining 8 days at LGB for $327. Total price: $645. Even cheaper if you structure your trip to fly midweek, AND you don't get to sit in potentially terrible traffic driving from LV to OC! It's typically a brisk 4hr drive, but my personal worst was 8 hours the Monday of President's Day weekend. Never again
If your plans best fit with a car, I would a) book with Costco, b) start a tracker with Autoslash, c) watch your email for periodic updates to your rate and book when it drops. This way, you at least have something, and if a better rate comes along, the process is automated. Goodluck!