Ok, we're all agreed on price level affecting numbers of visitors. But lets go to the next level here...
What exactly are you trying to accomplish, what are your goals for this zoo?
If this is a pure money making to build bigger, better, faster then you'll probably think you want to set the admission price higher. hmmmm, maybe not.
I've found the best strategy is to vary the admission price from $19 to $24, leaving the higher price admission to deliberately slow down the number of visitors.
The low price of $19 to start, goads more and more visitors through the gates of your "starter" zoo, and by a careful mix of exhibits, vendors and attractions, (and needless to add - a few best done exibits) your first visitors will quickly raise your zoo's popularity, spend more money, and donate like crazy.
If your goals for your zoo is an awards based zoo, the same strategy seems to work best there as well, getting you to the first level awards more quickly and still generating enough money to keep building.
I only raise ticket prices to help reach building goals more quickly,.. or as mentioned earlier, to temporarily, or more permanently decrease guest flow.
Hope this helps
Build on