It is based on music data and sound quality. Why don't you download Spotify music via AudKit Spotify downloader for offline listening? So that you don't need to use any mobile data.
If you listen to songs of high quality, it must cost you more data. So why not download the songs you usually listen to and convert to MP3 with Tunelf Spotibeat Music Converter. Then you can play Spotify songs on TracFone without data.