Harmonix has announced that a week long sale of over 1,100 songs will be on sale through all platforms in the Rock Band Store. The tracks will be 50% off as well the track packs. The sale will hit tomorrow, September 18th. This is really useful for people just now getting on the Rock Band Train or people who recently got Rock Band Blitz (as that game allows you to use all Rock Band tracks)
For a list of tracks that will be part of the sale and when they will be ready, check out this PDF.