Using Spotify every day is one of the biggest factors in my usage, but I never got over 500MB. Can you get Spotify on iPhone? One of the big bandwidth savers is that you can download your playlists to your phone, so the songs only have to be downloaded once (You can cache up to ~3000 songs like this). You can set it to only sync when on wifi, so you'll have all your songs when you're out and about (and underground!) without using so much of your 3G bandwidth.