I believe Rock Band's prices per song are 99p each, and you get 20 in this pack. Noticed as part of GAME's easter deals. From the site:
Rock Band Track Pack Volume 2 is a musical expansion to Rock Band and Rock Band 2, the smash-hit collaborative, competitive, and single player music game. Rock Band lets you and your friends form your own band and live out your rock and roll fantasies to musics greatest soundtrack.
Created by Harmonix, the leading developer of music-based games, Rock Band is the ultimate platform for gamers to experience music, giving players the chance to rock out in the shoes of the biggest guitarists, bassists, drummers and singers of all time as a solo superstar or as part of a hard rocking band. Built on unprecedented deals with top record labels and music publishers, the music featured in Rock Band spans all genres of rock and includes many master recordings from legendary artists.
Rock Band Track Pack Volume 2 is a standalone software product that allows owners of Rock Band Special Edition and Rock Band 2 Special Edition or Rock Band (1 or 2) peripherals to keep the party going with more hit songs from the pantheon of rock music, hand-picked with care by Harmonix.
Keep it Going! A fully functional game with all the great features of Rock Band, including: Solo and Band Tour Mode, Quickplay, Tug of War, Score Dual, and song unlock progression.
The Best Around! 20 of the most popular tracks in Rock Band history distilled to perfection onto one disc, all utilizing the original master recordings.
Go Beyond the Disc! X360 and PS3 software ships with download codes to import all of these tracks for use directly in Rock Band 1 and Rock Band 2.
The 20 song track list includes:
Airwaves & Angels - It Hurts
Avenged Sevenfold - Afterlife
Blondie - Call Me
Devo - Girl U Want
Disturbed - Indestructible
Duran Duran - Rio
Garbage - Why Do You Love Me
Fall Out Boy - This Aint a Scene
Judas Priest - You've Got Another Thing Comin'
Lynyr D Skynyr-D - Simple Man
Max? Park - Girls Who Play Guitars
M?y Cr? Saints Of Los Angeles
Nine Inch Nails - The Perfect Drug
Pixies - Monkey Gone To Heaven
Papa Roach - Time Is Running Out
Red Hot Chilli Peppers - Snow (Hey Oh)
Smashing Pumpkins - Zero
The Cars - Just What I Needed
The Police - Message In A Bottle
Weezer - El Scorcho