It is 16euros to enter the Palace and my friend and I would both highly recommend it ! It was absolutely marvelous and we never wanted to leave.
You follow a select route around the Palace and get to see each and every room, more beautiful than the next, with descriptive information detailing everything that you see (highly recommend reading these as they are informative and amusing descriptions).
Then, you are led into the garden and it is simply breathtaking. With views around the island and of the lake; peacocks strutting about the Palace grounds; flora and fauna that is as impressive as it is luscious. We wandered about the Palace and grounds for hours.
Give yourself at least an hour and a half to truly appreciate the entire Palace/grounds.