Dude, you walk into the zoo for free, no admission. I grew up in St Louis and the first time I saw the zoo it was horrid it was still the World's Fair exhibits. There was a huge fuss over it and rather than charge admission to raise funds, it was decided to levy a bond IIRC. Many times over the years since, every time the zoo announced they needed money and were going to have to charge admission, the public call was always "no, we'll do it publicly". Look at the records o the votes. Marlin Perkins was also good at bringing in funding.
I'd also be interested to see when the last time a new public funding issue was levied, I bet it's been a while. You'd be surprised at how many people in St Louis are "Zoo Parents".