The City's budget reflects funds utilized exclusively for the maintenance of zoological exhibits in Balboa Park which are financed from a fixed property tax levy ($0.005 per $100 of assessed valuation) as authorized by Section 77a of the City Charter. This fund is administered by the Department of Finance. cut and paste

Historically, entry fees paid the feeding and operating costs. Donations and estate giving paid for expansions, redo's, and the massive effort to prevent extinction around the world. The latter being the main function. This obviously is a material simplification of a huge international effort by the origination (my comments).

The San Diego Zoo Wildlife Alliance is a nonprofit organization headquartered in San Diego, California, that operates the San Diego Zoo and the San Diego Zoo Safari Park.[1] Founded in 1916 as the Zoological Society of San Diego under the leadership of Harry M. Wegeforth, the organization claims the largest zoological society membership in the world, with more than 250,000 member households and 130,000 child memberships, representing more than half a million people.[2] The San Diego Zoo and San Diego Zoo Safari Park feature a combined 15,000 animals of 750 species,[3] not all of which are displayed publicly. cut and paste

In the early 1970s, Dr. Kurt Benirschke, a Professor of Reproductive Medicine and Pathology at the University of California, San Diego, became chairman of the university's advisory committee to the Zoological Society dealing with research and animal reproduction.[52] He organized the committee to prepare a white paper describing the need for in-house research to examine the problems of breeding and sustaining endangered species populations in managed care.[52] In 1975 Benirschke and then-San Diego Zoo Director Charles Bieler established a new research department at the Zoo with Benirschke as its director.[9][52] Originally based in the Zoo's Biological Research Institute built 49 years prior, the department eventually expanded under the leadership of Dr. Allison Alberts to eight research- and education-centered divisions—Recovery Ecology, Plant Conservation, Population Sustainability, Conservation Genetics, Reproductive Sciences, Disease Investigations, Community Engagement, and Biodiversity Banking—employing over 200 scientists participating in more than 160 conservation studies and projects in 35 countries. cut and paste

The San Diego Zoo Wildlife Alliance is a nonprofit organization classified as 501(c)(3), making it tax-exempt according to the Internal Revenue Service.[2] The organization is funded through grants, membership revenue, and from sales of tickets, merchandise, and food at the Zoo and Safari Park.[65] Additionally, the San Diego Zoo Wildlife Alliance receives revenues from property taxes collected by the City of San Diego, the result of a proposition passed in 1934 that allows the organization to receive $0.02 from every $100 collected in property taxes, to be used for maintenance of zoological exhibits at the San Diego Zoo.[9][39][40] By 2015 this amounted to approximately $12 million a year, out of the organization's total annual earnings of almost $270 million.[40] The largest grant in the organization's history came in 2000: $7.5 million from the Arnold and Mabel Beckman Foundation, which went toward the construction of the Arnold and Mabel Beckman Center for Conservation Research on the Wild Animal Park property.[9][58] The largest individual donation to the organization came in 2004: $10 million from the estate of Joan Kroc, which was used in part to finance a renovation project at the Zoo which included a new habitat, Joan. cut and paste

We generate, share, and apply scientific knowledge vital to the conservation of animals, plants, and habitats worldwide. We help shape the vision of San Diego Zoo Wildlife Alliance to lead the fight against extinction. This is dated material, but general useful in understanding. cut and paste

My comment: Part of the purpose of the lower fees for "locals" is they are apt to visit the facilities more often (spend money), donate on specific projects and to include them in their estate planning. Many of us pay thousands of dollars a year for their membership. Note that the plants are more valuable than the displayed animals. There is a seed bank to help preserve plants. There is also the "frozen" zoo currently being used to bring back the Northern White Rhino, among many such efforts. Herds of animals are being amassed, stored and eventually released as circumstances allow in their home country/environment.