Personally, I think zoos should aim to find ways at reducing admission prices and generating revenue in other ways. Zoos have a number of missions, but two of the biggest are conservation education and community engagement. As educational and community institutions, zoos should aim to be as accessible to as many people as possible, as this would be the best way to support the mission of education, rather than only educating those who can afford to go. Even $30 a day, which may seem reasonable to many on here, would be $150 for a couple to take their three kids. For many families, spending $150 on a day outing to the zoo is prohibitively expensive, and that's before adding parking, food, etc. There are many kids in this country who have never visited a zoo, aquarium, or museum, and the reason isn't always that it's not something their family wants to do, but instead that it's not something their family can afford to do. It's just wrong in my eyes for an educational institution, which should be working as a community organization, should be inaccessible due to economics.
I would love to see zoos opt to, in a mission to be accessible to all, decrease their ticket prices and find other ways to generate revenue. Have extra special events, offer paid animal encounters/behind-the-scenes experiences more frequently, offer more additional cost attractions (train ride, 4-D movie, budgie feedings, etc.), have more avenues for people to donate money, charge extra for select exhibits, have a "suggested admission price" instead of a required admission price, I'm sure there are many more innovative ways at revenue generating as well that I just haven't thought of. Tickets should not be the primary way zoos, or aquariums and museums, are making money.