San Diego Zoo

This is one of the world's top zoos, attracting over 4 million visitors each year. It spans over 100 acres and features lush, naturalistic habitats for more than 3,700 different animals. Plan on spending the entire day here because there's plenty to see and do. A day pass costs $69 for adults and $59 for kids, and the experience is well worth the cost.

At the moment, a day pass costs a mere $ 58 - so not exactly cheap

zoo.sandiegozoo.org/

Located in Balboa Park, San Diego, California, the San Diego Zoo features over 3,700 animals from over 650 species and subspecies over 40 acres. It is privately and not-for-profit operated by the Zoological Society of San Diego. With approximately 3.2 million visitors per year, it is the most visited zoo in the USA. (Wikipedia)