Alysia Thomas, a stay-at-home mother in this working-class city, tells her children to skip a bath on days when they do not play outside; that holds down the water bill. Lillian Barrera, a housekeeper who travels 25 miles to clean homes in Beverly Hills, serves dinner to her family on paper plates for much the same reason. In the fourth year of a severe drought, conservation is a fine thing, but in this Southern California community, saving water means saving money.The challenge of California’s drought is starkly different in Cowan Heights, a lush oasis of wealth and comfort 30 miles east of here. That is where Peter L. Himber, a pediatric neurologist, has decided to stop watering the gently sloping hillside that he spent $100,000 to turn into a green California paradise, seeding it with a carpet of rich native grass and installing a sprinkler system fit for a golf course. But that is also where homeowners like John Sears, a retired food-company executive, bristle with defiance at the prospect of mandatory cuts in water use.