Unfortunately, this is by far not the only zoo with a high (ripp-off?) entrance fee.
San Diego Zoo is higher (with all the extras included most people don't need), although you get back a good value. Also many Australian Zoos, specially the "world famous" Australia Zoo of Steve Irwin, claim a nice amount. And here in Europe, Beauval and Amnéville are very expensive (more than 30 Euros). Not to mention the totaly overpriced London Zoo (25 pound = about 40 US-D).
Thing is, London Zoo is expensive by zoo standards, but actually pretty good value in terms of London attractions. The Madame Tussauds Waxworks, London Dungeon, London Eye, SeaLife London Aquarium and Tower of London are all £20 or more for an adult entrance, and the Zoo offers a much longer experience than any of these. (Most of the big museums are free of course, and are by far the best value in London!) The problem with London Zoo's entrance price is that London is expensive, not particularly the zoo.
Not sure what Whipsnade and Chester's excuses are!
...I jest! I think Whipsnade is just being priced (rightly or wrongly) to be comparable with London. Chester appear to be setting prices so as to try to spread the visitors more evenly over the year, with very high summer prices to almost deliberately discourage some visitors but cheaper options in winter to encourage them then.
