As I said to someone else...
People keep saying this and no one seems to have any evidence other than 'stands to reason', or 'must be the case'.
All businesses offer discounts in one form or another. At work I offered someone 5% off last week because he introduced us to another customer. No one else's price went up, we just took a very small dip on potential profit on that order. The result was we had 2 orders...which was better than 1 full price orders.
I'm not sure how many other ways I can explain that the cost of opening the zoo doesn't change whether 100 or 200 people visit - there's no lost revenue - only additional revenue from people that otherwise might not have visited.