It depends on where you want to go?
The cost depends on the class of car you select to travel in. Coach is the least expensive. The Coast Starlight now carries a Business Class car which is a bit more expensive but gives a guaranteed seat with more leg room. The sleeper cars (usually3) have roomettes that accommodate 1 person on the lower bed and 1 person on an overhead bunk. There are other sleeping accommodations that serve 3-4 travelers. The roomettes all share a bathroom down the hall and can use the shower and other facilities on the lower level. All meals are included in the sleeper car price. Prices vary so check the travel dates with Amtrak to get accurate pricing.
You asked about round trip from Los Angeles Union Station. Train 14 is the northbound Coast Starlight. The scenery in Northern California and the Oregon Cascades is spectacular. The southbound Coast Starlight train number 11 goes thought the cascades at night so it is not very scenic.
Get quick answers from Amtrak Coast Starlight staff and past visitors.