Ghost producing means creating a track for another artist/DJ with them having little to no participation in the creative process. Often the ghost producer stays completely anonymous and is not mentioned in the credits nor receives any publishing rights.
Ghost produced songs normally cost between $100-10,000+ per track depending of course on the quality, style and potential of the track.
Ghost producing is not limited to the Electronic Music Scene, though here it is definitely more frowned upon since the producer of a track normally is also the face of the music. Compared to other music genres, like Pop, where the (ghost) producers are behind the scenes to begin with.
Famous ghost producers include Hardwell and Martin Garrix.