How much does it coast to have a party at the park and how do i go about booking a spot?

You need to contact the bureau of parks and recreation. They're online.

He is correct. It also will vary depending on what you want.

You can probably go to Parks and People near Druid Hill Park

There is no cost for the park, the park is free like any other (traditional) park. Just hope that the park is not full, or has no empty space for partying.