This is about as cheap you can go and still end up in a decent and central place. For around 27 EUR you can sleep in a 6 bed shared room with its own shower and (seperate) toilet. There are also private rooms available.
The giant hostel has been updated and renewed a few years ago, that's why it is modern and has all the facilities that you might expect (laundry, internet, lockers, pool table, etc.)
Most people rate this hostel as a great one and would stay again, but some have negative experiences (mainly unfriendly experience with the staff). Many people appreciate the breakfast.
The Vondel park is located near the night life and shopping district Leidse square and very close to the museum quarters where the Van Gogh and Rijks museum can be found.