The sleeper cars do cost extra and they’re priced in different tiers—I had the smallest, the roomette. They can be pricey (I booked far in advance) and the price is dependent on both demand and the length of your trip. One of the greatest advantages of having a sleeper is having all meals included on your trip. Since you’ve already booked your ticket though, I recommend asking the conductor how much an upgrade to a sleeper would cost once you’re aboard the train. I’ve never done it myself but I’ve talked to people who have and they’ve gotten really great deals that are much cheaper than what’s offered online.
The California Zephyr is a phenomenal train ride and it looks like you’ll be going through the Sierra Nevada Mountains. If you don’t end up with a sleeper, get cosy in the Observation Car! Have fun!