Regular itunes downloads are 128kbps and 79p each, some tracks are also available as "iTunes Plus" which is 256kbps and 99p each - also note that iTunes downloads are mp4, not mp3 so should in theory be slightly better than mp3 as the codec is newer.
Also, fixed bitrate however large is inefficient anyway - LAME APS rules or if you have to go uncompressed, theres always Apple Lossless or FLAC.