The price depends greatly on when you visit the park. It is cheapest between November 1 and March 31st (the entrance fee for an adult in this period is currently 10 euros). The most expensive period is during the peak months (June 1st to September 30th) and then the adult entrance fee is between 27 euros and 40 euros (depending on what time of the day you decide to visit the park).
Students and children can visit Plitvice at discounted prices.