I doubt that Spotify is charging money for inclusion in the Discover Weekly playlists. This would run counter to the goals of promoting Discover Weekly usage, which is to get more users thinking "What's new on Spotify this week?" rather than "Let me pull up the new Taylor Swift album." The more that users flock to Discover Weekly, the more leverage that Spotify gains against the labels, so my guess is that they're striving for a wide diversity of rightsholders in Discover Weekly so that any one label threatening to leave will not damage the playlist too badly.